Polyamide 6 based block copolymers synthesized in solution
and in the solid state
Citation for published version (APA):
Cakir, S. (2012). Polyamide 6 based block copolymers synthesized in solution and in the solid state. Technische Universiteit Eindhoven. https://doi.org/10.6100/IR730916
DOI:
10.6100/IR730916
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Polyamide 6 Based Block Copolymers
Synthesized in Solution and in the Solid State
PROEFSCHRIFT ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de rector magnificus, prof.dr.ir. C.J. van Duijn, voor een commissie aangewezen door het College voor Promoties in het openbaar te verdedigen op maandag 19 maart 2012 om 16.00 uur door Seda Çakır geboren te Giresun, Turkije
prof.dr. C.E. Koning
Polyamide 6 based block copolymers synthesized in solution and in the solid state by Seda Çakır Technische Universiteit Eindhoven, 2012 A catalogue record is available from the Eindhoven University of Technology Library. ISBN: 978‐90‐386‐3113‐4 Copyright © 2012, Seda Çakır Cover design: Taylan Çakır Printed by: Proefschriftmaken.nl || Printyourthesis.com Published by: Uitgeverij BOXPress, Oisterwijk
“Ya ölü yıldızlara hayatı götüreceğiz ya da dünyamıza inecek ölüm.” “Either we bring life to the dead stars or the death will descend to earth.” Nazım Hikmet
To my mom, dad, Taylan and İso…
Table of contents
Chapter 1 General Introduction 1.1 Introduction to polyamides and polyamide 6 2 1.2 Crystal structure of polyamide 6 5 1.3 Modification of polyamides 8 1.4 Modification of polyamides by solid‐state polymerization 9 1.5 Objectives and outline of the thesis 12 References 13 Chapter 2 Partially Degradable Polyamide 6‐Polycaprolactone Multiblock Copolymers 2.1 Introduction 18 2.2 Experimental 20 2.2.1 Materials 20 2.2.2 Synthesis of diamine end‐capped PA6 21 2.2.3 Synthesis of hydroxyl end‐capped oligoester 21 2.2.4 Synthesis of diisocyanate end‐capped polycaprolactone 22 2.2.5 Copolymer synthesis 22 2.2.6 Enzymatic and non‐enzymatic hydrolysis 22 2.2.7 Characterization 23 2.2.7.1 Size Exclusion Chromatography (SEC) 24 2.3.7.2 Nuclear Magnetic Resonance (NMR) Spectroscopy 24 2.3.7.3 Differential Scanning Calorimetry (DSC) 24 2.3.7.4 Fourier Transform Infrared (FTIR) Spectroscopy 24 2.3.7.5 Potentiometric titration 24 2.3.7.6 Scanning Electron Microscopy (SEM) 25 2.3 Results and Discussion 25 2.3.1 Diamine end‐capped PA6 26 2.3.2 Hydroxyl and diisocyanate end‐capped polypropylene adipate 30 2.3.3 Diisocyanate end‐capped PCL (TPCL) 32 2.3.4 Multiblock copolymers of PA6C and TPCL 34 2.3.5 Hydrolytic and enzymatic degradation of PEA‐ASM 39 2.4 Conclusions 42 References 43 Chapter 3 Multiblock Copolymers of Polyamide 6 and Diepoxy Propylene Adipate Obtained by Solution and Solid‐State Polymerization 3.1 Introduction 46 3.2 Experimental 48 3.2.1 Materials 3.2.2 Model reactions of glycidyl phenyl ether and propanoic acid 48 3.2.3 Synthesis of fully carboxyl end‐capped polyamide 6 48 3.2.4 Polyamide 6‐poly(propylene glycol) diglycidyl ether model reactions 49 3.2.5 Polyamide 6‐diepoxy propylene adipate reactions 49 3.2.6 Characterization 503.2.6.1 Size Exclusion Chromatography (SEC) 50 3.3.6.2 Nuclear Magnetic Resonance Spectroscopy (NMR) 50 3.3.6.3 Differential Scanning Calorimetry (DSC) 50 3.3.6.4 Thermogravimetric Analysis (TGA) 51 3.3.6.5 Potentiometric titration 51 3.3 Results and Discussion 51 3.3.1 Model reactions with glycidyl phenyl ether and propanoic acid 51 3.3.2 Model reactions with poly(propylene glycol) diglycidyl ether (PPGE) and PA6 54 3.3.3 Diepoxy propylene adipate (DEPA) and PA6 reactions 63 3.4 Conclusions 70 References 71 Chapter 4 Incorporation of a Semi‐Aromatic Nylon Salt into Polyamide 6 by Solid State or Melt Polymerization 4.1 Introduction 74 4.2 Experimental 75 4.2.1 Materials 75 4.2.2 Dytek A‐isophthalic acid salt preparation 76 4.2.3 Solution mixing of PA6/Dytek A‐IPA nylon salt in HFIP 76 4.2.4 Solid‐state polymerization (SSP) 76 4.2.5 Melt polymerizations 78 4.2.5.1 Caprolactam (CL)/Dytek A‐IPA salt 78 4.2.5.2 Dytek A‐IPA homopolymer 78 4.2.6 Characterization 79 4.2.6.1 Size Exclusion Chromatography (SEC) 79 4.2.6.2 Differential Scanning Calorimetry (DSC) 79 4.2.6.3 Nuclear Magnetic Resonance (NMR) Spectroscopy 79 4.2.6.4 Potentiometric titration 80 4.3 Results and Discussion 80 4.3.1 Low molecular weight PA6/Dytek A‐IPA copolyamides via SSP and MP 83 4.3.1.1 Molecular characterization of PA6/Dytek A‐IPA copolyamides by 1H NMR, SEC and titration 83 4.3.1.2 Thermal properties of PA6/Dytek A‐IPA copolyamides 90 4.3.2 High molecular weight PA6/Dytek A‐IPA copolyamides via SSP and MP 93 4.3.2.1 Molecular characterization of PA6/Dytek A‐IPA copolyamides 93 4.3.2.2 Sequence distribution and degree of randomness analysis by 13C NMR 96 4.3.3.3 Thermal properties of the copolyamides prepared with limited Dytek A loss 101 4.4 Conclusions 107 References 108
Chapter 5 Investigation of Local Chain Conformation and Morphology of Polyamide 6 Modified by a Semi‐Aromatic Nylon Salt 5.1 Introduction 112 5.2 Experimental 114 5.2.1 Wide Angle X‐Ray Diffraction (WAXD) 114 5.2.2 Fourier Transform Infrared (FTIR) Spectroscopy 114 5.2.3 Solid State NMR 115 5.3 Results and Discussion 115 5.3.1 WAXD studies 116 5.3.2 FTIR analysis 117 5.3.3 Solid State NMR analysis 121 5.4 Conclusions 126 References 126 Chapter 6 Epilogue and technology assessment 129 Appendix 133 Summary 137 Acknowledgements 141 List of publications 145 Curriculum vitae 146
Glossary
α Alpha form γ Gamma form ∆HC Enthalpy of crystallization ∆Hm Enthalpy of melting [NH2] Amine end group content [COOH] Carboxylic acid end group content 1H NMR Hydrogen‐1 nuclear magnetic resonance spectroscopy 13 C NMR Carbon‐13 nuclear magnetic resonance spectroscopy AA Adipic acid ACA 6‐Aminocaproic acid ATR Attenuated total reflectance C Concentration or carbon CDCl3 Deuterated chloroform CL ‐Caprolactam CP/MAS NMR Cross‐polarization magic angle spinning NMR spectroscopy D2O Deuterium oxide DBD Dibutyltin dilaurate DEPA Diepoxy propylene adipate DMA Dimethyl adipate DMAP 4‐Dimethylaminopyridine DMSO Dimethyl sulfoxide DSC Dynamic scanning calorimetry DyI Dytek A‐isophthalic acid salt Dytek A 1,5‐diamino‐2‐methylpentane FTIR Fourier transform infrared spectroscopy GPE Glycidyl phenyl ether HCl Hydrochloric cid HFIP 1,1,1,3,3,3‐Hexafluoro‐2‐propanolIPA Isopropanol or isophthalic acid L Number average block length Mn Number average molecular weight Mw Weight average molecular weight meq Milliequivalent MP Melt polymerization Mp Peak maximum MW Molecular weight p Conversion PA Polyamide PBS Phosphate buffered saline PCL Polycaprolactone diol PD 1,3‐Propane diol PDI Polydispersity PE Polyester PEA Polyesteramide PEA‐ASM Polyesteramide after solution mixing PPA Polypropylene adipate or propanoic acid PPGE Poly(propylene glycol) diglycidyl ether p‐XDA p‐Xylylenediamine r Ratio of the reactants R Degree of randomness RT Room temperature SEC Size exclusion chromatography SEM Scanning electron microscopy SH Salt homopolymer SSP Solid‐state polymerization T5% Temperature at 5% weight loss Tc Crystallization temperature Tg Glass transition temperature
Tm Melting temperature TBO Titanium(IV)butoxide TDI Toluene 2,4‐diisocyanate TEA Triethylamine TFE 2,2,2‐Trifluoroethanol TGA Thermogravimetric analysis THF Tetrahydrofuran TPCL Toluene diisocyanate end capped polycaprolactone VT Variable‐temperature WAXD Wide angle X‐ray diffraction Xc Percent crystallinity Xn Degree of polymerization
CHAPTER
1
G
ENERAL
I
NTRODUCTION
Summary
In this chapter a general introduction to polyamides and specifically to polyamide 6 is given. Synthetic techniques for the production of PA6 as well as its crystal structure are described. Possible modification techniques of the polyamides are covered and modification by solid‐state polymerization is discussed in detail. Finally, the objectives and the outline of this thesis are explained.
1.1 Introduction to polyamides and polyamide 6
Polyamide, in its fully aliphatic form also known as nylon, is the first commercial synthetic polymer entering modern life. The chemical structure is similar to that of proteins and polypeptides such as silk and wool, which are formed by the coupling of amino acids in nature. Polyamides have a repeating amide group (–CONH–) in their molecular structure and the type of the repeating unit determines the properties of the polyamides. The structure of the amide bond and the chain dimensions are represented in Figure 1.1 as postulated by Flory in 1953.1 The first commercial polyamide was invented by the research group of Wallace Carothers at Du Pont in 1935 and was presented as the world’s first synthetic fiber. The polymer was called Nylon 66 (PA66) because of the six carbons in the diamine and respectively the diacid residues.2 So, here the repeat unit consists of two monomeric residues. The reaction for the preparation of PA66 is shown in Figure 1.2. Commercialization of Nylon 66 replaced the usage of silk and in first instance it was used for military supplies such as parachutes, vests, tires and ropes.
Figure 1.1 Structure and the dimensions of the amide group in aliphatic polyamides.
Figure 1.2 Reaction scheme for the synthesis of Polyamide 66.
A few years after the invention of PA66, in 1938, Paul Schlack and his co‐workers at IG Farben were able to make a polyamide out of one starting material which was named
Polyamide 6 (PA6).3 In this case the ‘6’ stands for the total number of carbon atoms present in the single amino acid residue representing the repeat unit. In 1940 the first polyamide stockings were introduced to the American market. Up till 1950 almost the total polyamide market consisted of PA66. Thereafter PA6 slowly found its place.4 Later on, many other polyamides were introduced to the market such as PA69, PA610, PA11, PA12 and PA46 as well as aromatic polyamides. The type of polyamide based on amino acids is called an AB polymer, whereas a polyamide based on diamines and dicarboxylic acids is a polymer of the AABB type.
The most common synthetic technique for the preparation of PA6 is the hydrolytic ring opening polymerization of ε‐caprolactam (CL) at 250‐270 °C. This technique consists of three equilibrium reactions as shown in Figure 1.3. The first step involves the hydrolysis of CL forming ε‐aminocaproic acid followed by the direct addition by ring opening polymerization (ROP) of CL to the amine end group of a growing chain (which can also be the ε‐aminocaproic acid). Finally, the polycondensation reaction between the amine and carboxylic acid end groups leads to high molecular weight product where water is released. In practice, the ROP and the polycondensation reaction occur simultaneously during a significant part of the process.
Figure 1.3 Hydrolytic ring opening polymerization of ε‐caprolactam for the synthesis of Polyamide 6. Hydrolytic ring opening of ε‐caprolactam (1), addition reaction of ε‐ caprolactam to a growing chain, the CL ROP (2) and polycondensation reaction between the end groups (3).
The PA6 polymerization consists of equilibrium reactions and at the polymerization temperature around 260 °C at the end of process there are always around 10 wt% unreacted CL and cyclic oligomers present. These cyclic compounds, mainly CL, are formed by back biting reactions. Therefore, these low molecular weight extractables are removed by extraction with water after the reaction.
PA6 is predominantly produced by a continuous multi‐step process in industry as schematically shown in Figure 1.4.5, 6 CL and water enter the top of the VK (Vereinfacht Kontinuierlich) tube, which operates at about 250 °C and 1 atm. As the polymer forms it moves down the column with increasing viscosity and a mixture of polymer, unreacted monomer, water and water soluable oligomers exits the bottom of the VK tube. This mixture enters a pelletizer, and the pellets containing extractables enter the top of a hot‐ water leacher. Water and the product stream of the VK tube flow countercurrently to remove caprolactam and oligomers from the polymer pellets. Finally, the extracted pellets enter a solid‐state polymerization reactor. Dry nitrogen gas entering the bottom of the reactor increases the temperature and drives the reaction equilibrium forward, leading to the formation of higher molecular weight polyamide 6 (Mn=24‐32 kg/mol).
Figure 1.4 VK tube process for PA6 production.5
Today, PA6 and PA66 continue being the most widely produced commercial products among all polyamides accounting for 90% of the nylon manufactured globally (3.4 x 106 ton/year).7 Nylon has replaced metal for mechanical performance by serving as an engineering plastic with good stiffness, strength, toughness, resistance to chemicals and thermal stability. The chemistry and properties of polyamides and specifically PA6 were well described by several authors.6, 8‐10 PA6 is mostly used for automotive, electrical and packaging applications. Additives used during the production provide end‐products for various applications. Drawback of PA6 is the relatively high moisture absorption (9.5% at 100% relative humidity and 22 °C), which results in a plasticizing effect and enhances toughness due to the drop of the glass transition temperature to a value below room temperature.6
1.2 Crystal structure of polyamide 6
Polyamides are semi‐crystalline polymers having regular crystalline lamellae separated by amorphous regions at room temperature. Semi‐crystallinity of polymers is desired for many applications where the crystalline part provides strength, stiffness and chemical resistance and the amorphous region provides flexibility and toughness. One of the main characteristics of the polyamides is the ability of the –N–H group to form strong intra and intermolecular hydrogen bonds with the –C=O group in the amide linkages within the same or neighboring chains. The chains are oriented in a way to maximize the hydrogen bonding which also provides high regularity (Figure 1.5.c).11‐13 The character of the hydrogen bonds and the electrostatic attraction between the electric dipoles contribute to the strength of the amide‐amide interactions.14 During the glass transition around 47‐57 °C dipolar interactions are broken, whereas during the melting process at 220‐223 °C most of the hydrogen bonds are broken.15
Although during the early years of polymer science polymer crystals were believed to be formed according to the fringed micelle model, Keller in 195716 showed that polymer chains are folding back and forth on themselves where folds occur at the faces as shown in Figure 1.5.a according to his electron diffraction experiments. This model was called
the “adjacent re‐entry model” and was shown to be more predominant for solution‐ grown crystals than for crystals grown from the melt. Low molecular weight polymers tend to fold into this structure as well.17 This model is also divided into two different forms: the smooth surface model or the rough surface model where there is a sharp boundary between the crystal and the amorphous phase in the former model while large variations in the fold length may exist in the latter one.18 Later Flory suggested that a “switchboard model” is more probable for melt‐grown crystals where chains are randomly folding back into the same lamellae as shown in Figure 1.5.b.19 In this model the amount of adjacent re‐entry is small since the conditions are far from equilibrium so that adjacent folding depends on molecular weight and molecular architecture.20, 21 The driving force for the chain to uncoil from a high entropy conformation is the lowering of the enthalpy due to the formation of favorable secondary H‐bonding interactions. The extent to which a polymer will crystallize is determined firstly by thermodynamic forces favoring maximum potential crystallinity at equilibrium, and secondly by the kinetic forces determining the rate and extent to which the polymer may actually approach such a theoretical maximum degree of crystallinity. Thermodynamic forces that can be mentioned are regularity, symmetry, even or odd number of atoms in the monomeric unit, polarity and branching, while the kinetic forces include molecular flexibility and processing conditions.21
Figure 1.5 Two main fold models of polymer crystals: adjacent re‐entry model (a), switchboard model (b) and intramolecular hydrogen bonding in PA6 (c).
In the most ideal PA6 crystallization case, i.e. from solution, chain folding and the formation of hydrogen bonds occur in lamellar sheets, named β‐sheets, as shown in Figure 1.5. The lowest enthalpy level for a folded molecule results in intramolecular hydrogen bonding which is only formed within the sheets. The sheets are connected to each other by van der Waals interactions. The most stable crystal packing for PA6 is called the “α” form. This phase consists of molecules in an extended chain conformation with hydrogen bonds between anti‐parallel chains (see anti‐parallel orientation in Figure 1.6.a). In this case within each β‐sheet all possible H‐bonds can be formed without any problem, which is why this crystal form is the most stable one. In the second form, which is less stable and is called the “γ” form, the chains within one β‐sheet are oriented in the parallel form (Figure 1.6.b) and complete H‐bonding is only possible if the chains are somewhat distorted. The amide groups are twisted out of the plane of the methylene groups, shortening the chain repeat distance and permitting intermolecular hydrogen bonding between the parallel chains.11, 22‐27 Both forms are shown in Figure 1.6.
1.3 Modification of polyamides
In most cases polyamides are modified for industrial applications to end up with better properties in line with the desired applications. In this way, properties of the bulk polyamide can be modified to yield more flexibility, longer pack life, increased glass transition temperature, lower melting temperature, higher thermal/solvent/abrasion resistance, enhanced flame retardancy, improved shrinkage and mechanical properties, etc. All of these improvements can usually be obtained without following expensive production routes.
The most common technique used for this modification is to copolymerize the standard monomers of a specific polyamide with desired comonomers in the melt by which a random distribution of the property‐changing comonomers is obtained. Another technique is blending the specific PA with a polymer improving the desired properties where the components are mixed only to some level to make a physical mixture. If a physical mixture of two step‐growth polymers is held in the molten state, interchain reactions can take place yielding block‐like copolymers which will convert into a totally random microstructure as the reaction proceeds.
For instance a melt reaction of AB type monomers with AA and BB type monomers will result in a copolyamide with both AB and AABB type structures. However, depressions in melting and crystallization temperatures to below the original values of both polymers are obtained in the end.28‐31 This behavior is well described by Flory32 and Jo et al.33 theoretically. This depression might be prevented by blending two types of homopolyamides for just a sufficient time, or by sequential addition of monomers and preventing transamidation reactions, by which block‐like copolymers can be obtained.34, 35 The advantage of such blocky structures is that the physical properties of both original polyamides are still present in the final material, whereas a completely random copolyamide might lose the crystallinity and favorable physical properties of both blend components.
Copolymerization of polyamides with non‐amidic units is also possible and widely used to make copolymers like poly(ester amide)s, poly(ether amide)s, poly(urea amide)s and poly(urethane amide)s where the strength, crystallinity and thermal stability of the polyamide can be combined with the desired properties of the other polymer type by the addition of the other components. Polyesteramides have gained much interest, mainly due to enhanced biodegradability by the incorporation of ester linkages. Polyamides are well known to be highly resistant to biodegradation in nature; however, it has been shown that the combination with aliphatic ester groups makes it liable to hydrolytic and enzymatic degradation. Preparation of biomaterials for tissue engineering or drug delivery is also possible by this method.36‐40 Different synthetic approaches such as ring opening polymerization, ester‐amide interchange reactions, anionic polymerization, interfacial polymerization and polycondensation in the melt can be used.41‐56 It is also possible to enhance properties like lower moisture absorption and better dimensional stability by incorporating polyesters such as polyethylene terephthalate (PET).57‐63 Thermoplastic polyether‐block‐amides (PEBA) elastomers are also an interesting class of copolymers where hard segments consisting of crystallizable polyamide blocks provide the strength and the soft ether blocks provide the flexibility. In these PEBAs hard segments can interact with each other by hydrogen bonds.64‐67 Preparation of poly(urea amide)s and poly(urethane amide)s give the possibility to obtain polyureas or polyurethanes with improved thermal, mechanical and solvent resistance 68‐72 or dendritic self‐assembly structures.73, 74
1.4 Modification of polyamides by solid‐state polymerization
Solid‐state polymerization (SSP) implies heating the starting material, being either dry monomers or the prepolymer, at a temperature above the glass transition temperature but below the melting temperature , so that the mobile reactive groups are able to react but the material does not become sticky or a fluid. By‐products are removed by passing inert gas through the reaction medium or by maintaining reduced pressure. If SSP is performed starting with dry monomers it is referred to as direct SSP, whereas the latter is
called post‐SSP (or solid state postcondensation). Although SSP can be used for chain‐ growth polymers in industry it is mainly used for polyamides and polyesters. It is for example an important finishing technique to obtain high molecular weight polyamides (Mn > 25 kg/mol) suitable for spinning, extrusion and injection.75
The kinetics and the influence of various parameters involved in the SSP reactions of polyamides75‐83 and polyesters76, 81‐84 have been investigated by several research groups until now. There are four main steps governing the rate of SSP:75, 82
i) The intrinsic kinetics of the chemical reaction where the reaction temperature and the presence of catalyst are the most important factors. ii) The diffusion of the reactive end groups which is mainly dependent on the
reaction temperature, initial prepolymer molecular weight and crystallinity. iii) The diffusion of the condensate in the solid reacting mass which is affected
by the reaction temperature, particle size, gas flow rate and the presence of the catalyst.
iv) The transfer of the condensate from the reacting mass surface to the inert gas. Similar parameters as in the previous item (iii) are important.
The intermolecular exchange reactions involved in the SSP of polyamides are acidolysis, aminolysis and amidolysis reactions as shown in Figure 1.7.75, 85 Acidolysis is the reaction between an alkyl carboxyl group and an amide linkage, aminolysis is the reaction between an alkyl amine and an amide group, whereas the amidolysis is the reaction between two amide groups. All these reactions result in linear products such as polyamides, oligomers and by‐products. On the other hand, intramolecular reactions result in the formation of cyclic compounds.
Figure 1.7 Exchange reactions of polyamides: acidolysis (1), aminolysis (2), amidolysis (3). Possible side reactions observed after long reaction times during SSP of polyamides involve the formation of a secondary amine group from the reaction of two amine end groups which, after the reaction with a carboxyl end group, forms branched structures in the case of an AB type polyamide (like PA6) and crosslinked structures in the case of an AABB type of PA. Crosslinking is especially observed in case of PA66.86 During the SSP of PA46 the formation of high molecular weight polymers is inhibited by pyrrolidine formation, which is a chain stopper (Figure 1.8).87 The reaction of pyrrolidine end groups with water results in carboxyl end‐capped polymer chains which act as terminating agents.
Figure 1.8 Pyrrolidine end‐group formation and its reaction with water to form carboxyl‐ terminated chains. SSP is a very efficient and mild technique not only to reach high molecular weight step‐ growth polymers without having too many side reactions or without suffering from a very high melt viscosity, but also to incorporate other monomers/polymers into the main chain of the step‐growth polymer. As discussed in the previous section, most of the modification techniques for semi‐crystalline polymers lead to randomization by which the crystalline
phase is deteriorated, and as a result, mechanical and physical properties are reduced. However, SSP gives the possibility to modify step‐growth polymers by transreactions (see Figure 1.7) without the entire deterioration of the crystalline behavior. Previously, a three phase model has been proposed for semi‐crystalline polymers which consists of a crystalline fraction, mobile amorphous fraction (MAF) and rigid amorphous fraction (RAF).88, 89 During the SSP reactions, it is expected that only the mobile amorphous phase takes part in the aminolysis, acidolysis and amidolysis reactions so that the crystalline phase remains intact. This modification is represented in the picture in the first pages of this chapter and in Chapter 4. This concept accordingly should result in a block copolymer structure with crystalline homopolymer blocks and chemically modified and usually amorphous copolymer blocks. By this way, comonomers/polymers can be incorporated into PA6 backbone in the solid state and the resulting copolymers can retain their high melting temperatures, crystallization rates and good mechanical/physical properties. Recently, Jansen et al.89‐93 and Sablong et al.94, 95 studied the incorporation of diol monomers into poly(butylene terephthalate) (PBT) above Tg but below the melting temperature of PBT. Jansen and coworkers showed for the first time that copolyesters with non‐random distributions and high molecular weights were obtained after solution mixing of both components in a common solvent followed by subsequent removal of the solvent and SSP. Comparison with melt‐polymerized samples proved the superior properties obtained after the modification by SSP. Molecular and morphological structures were studied in detail via SEC, DSC, 1H NMR and 13C NMR and blocky microstructures were indeed confirmed after SSP reactions.
1.5 Objectives and outline of the thesis
The objective of the work described in this thesis is to chemically modify polyamide 6 (PA6) for realizing enhanced properties by solution and/or solid‐state polymerization in such a way that good material properties can be retained. One of the aims is to make partially degradable PA6 by incorporating hydrolyzable ester groups into the backbone of PA6. This can be done either in solution or in the solid state, depending on the functional
end groups which connect the short polyamide and oligoester/polyester blocks together. In this way multiblock copolymers of polyamide‐polyester can be prepared so that degradability is obtained in addition to the good properties of PA6. Chapter 2 describes the incorporation of diisocyanate end‐capped polyester into amino end‐capped PA6 in solution, whereas in Chapter 3 the incorporation of an epoxide end‐capped oligoester into carboxylic acid end‐capped PA6 is reported. Another aspect is to show that high molecular weight PA6 can be modified below its melting temperature by selective incorporation of a nylon salt where the salt is only incorporated in the amorphous phase, excluding the large crystalline fractions from the transreactions. For this purpose, as described in Chapter 4, a semi‐aromatic nylon salt with an irregular structure was chosen so that it cannot co‐ crystallize with the crystallizable PA6 segments and can be easily forced into the amorphous phase. It was shown that incorporation of the nylon salt into the amorphous phase via intermolecular exchange reactions without the deterioration of the crystalline phase is indeed possible. The effects of salt composition, reaction temperature and reaction time were investigated. Detailed characterization in terms of molecular weights, thermal properties and blockiness were performed. Morphological changes obtained after the SSP reactions via heating up to the melting temperature of the blocky copolyamide are also presented in Chapter 5. The thesis ends with a technology assessment (Chapter 6), describing the possible industrial implementation of the promising SSP concept for PA6 modification. References 1. Flory, P. J. Statistical Mechanics of Chain Molecules. Wiley‐Interscience: New York, 1969. 2. Carothers, W. H. US 2071250, 1937. 3. Schlack, P. US 2241321, 1941.
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CHAPTER 2
P
ARTIALLY DEGRADABLE POLYAMIDE 6‐
P
OLYCAPROLACTONE
M
ULTIBLOCK
COPOLYMERS
Summary Low molecular weight polycaprolactone was successfully incorporated into
polyamide 6 by solution and solid‐state polymerization after synthesis of both components with desired co‐reactive end groups. The structure and thermal properties of polymers before and after incorporation were analyzed by SEC, FTIR, NMR, titration analysis as well as TGA and DSC. DSC data, together with an increase in molecular weight pointed to a multiblock structure with almost maintained melting temperatures with respect to pure components. Degradation of polymers was performed via enzymatic and hydrolytic routes at 25 °C and followed by weight loss analysis, SEM and SEC.
2.1 Introduction
Polyamide 6 (PA6) is a high‐performance engineering plastic used for a wide range of applications in everyday life. Strong hydrogen bonding between the chains and high regularity in the crystalline phase provide excellent thermal and mechanical properties but on the other hand result in a highly resistant material to biodegradation in nature. As there is an increasing demand for disposable packaging applications the biodegradability of PA6 could be enhanced by incorporating hydrolyzable groups into the main chain. These hydrolyzable groups can be selected from various aliphatic polyesters which are well known to be biodegradable due to cleavable ester links. Polycaprolactone (PCL) is one of these polyesters which can be used both for biomedical and ecological applications.1‐3 As a special class of biodegradable polymeric materials, the synthesis and characterization of poly(ε‐caprolactam‐co‐ε‐caprolactone) copolymers have been studied by different research groups. Synthetic approaches include ester‐amide exchange reactions, anionic polymerization, interfacial polymerization, ring opening and polycondensation reactions.4‐ 14 Most of these works showed that the resulting copolymers have a random structure, whereas only a few papers described di‐ or tri‐ block structures. Degradation studies were also described in several articles5, 9, 11, 15, 16 by using different methods proving that these type of copolymers are susceptible to degradation, although mostly enzymatically. If the ester groups are randomly introduced into the PA6 main chain, the crystallization behavior of PA6 will be negatively affected, the melting temperature will be significantly reduced and the mechanical and physical properties, crucial for packaging applications (such as barrier properties), will become worse. This fact is also seen in the literature covered above where there is a big decrease in melting temperatures as the amount of ε‐ caprolactone increases when random copolymers are prepared. To the best of our knowledge well‐defined multiblock copolymers of this type of polyesteramides have not been synthesized yet and have certainly not been tested as biodegradable materials.
A promising synthetic method can be incorporating these degradable groups or blocks into the amorphous part of a relatively low molecular weight PA6 below the melting temperature of the PA6 crystals.17, 18 For this purpose, well‐known synthetic techniques can be applied to prepare this new type of PA6‐PCL block polymers by making use of isocyanate‐amine reactions at low temperatures. Until now D’Hollander et al.19 obtained shape memory polyurethane networks based on a triblock copolymer made by the reaction of isocyanate end‐capped PCL and excess of amine end‐capped poly(propylene oxide). Lee et al.20 prepared shape memory polyamides by linear chain extension of PCL and diamine‐terminated polyamide in the presence of hexamethylene diisocyanate (HDI). Their aim was to have shape recovery by using high fractions of (PCL‐HDI)n units (70%) compared to (polyamide‐HDI) units. According to the thermal analysis of the copolymers the highest melting temperature of the polyamide segments was 183 °C. It should be realized that the synthetic route used by Lee et al. results in a rather ill‐defined structure, since HDI can couple either two PA blocks, two PCL blocks or one PA and one PCL block. The aim of this chapter was to make well‐defined PA6‐based multiblock copolymers where the good properties of PA6 such as high melting temperature and crystallinity can still be maintained, whereas biodegradation can be an additional property. We followed a stepwise technique where low molecular weight amine end‐capped PA6 and isocyanate end‐capped PCL polymers were synthesized separately followed by solution and solid state step‐growth copolymerization of these telechelic building blocks at reduced temperatures. The relatively low temperatures should prevent aminolysis of the PCL ester groups by the PA6 amine end groups. In this way, the targeted reasonably well‐defined multiblock copolymers of PA6 and PCL could be obtained with PA6‐like thermal properties and partial biodegradability. We realize that the PA6 blocks are not degradable, but by degrading the PCL blocks the material may disentangle and fall apart into small fragments. Molecular weights of the synthesized polymers were characterized by using SEC, NMR and titration methods. SEC was also used as a useful tool to follow the reactions with time. Molecular structures of the products were investigated by FTIR spectroscopy. Thermal
analysis was performed by using TGA and DSC. Hydrolytic and enzymatic degradations were done in PBS buffer solution followed by surface analysis of the films by using SEM. HN 250 °C, 3 bar, 6 hours H2O O NH2 H2N HO O O O OH O O n n 65 °C, in THF, 2 hours OCN H3C NCO
PA6 (Mn=2,500 g/mol, titration) Diisocyanate end capped PCL
(Mn=1,850 g/mol, titration)
(CL) Polycaprolactone (PCL, Mn=1,600 g/mol)
[PA 6-b-PCL]nmultiblock copolymers
DBD O O O O O O O n H N HN O nO OCN H3C NCO CH3 H N H2N N H NH2 O O n m Caprolactam N H C O N H
Figure 2.1 Schematic drawing of stepwise synthesis of polyamide 6‐polycaprolactone multiblock copolymers obtained by solution and solid‐state polymerization.
2.2 Experimental
2.2.1 Materials‐Caprolactam (CL) was kindly provided by DSM. p‐Xylylenediamine (p‐XDA, >98 %) and toluene 2,4‐diisocyanate (TDI, >98 %) were purchased from Fluka. 1,3‐propane diol (PD), dimethyl adipate (DMA) and titanium(IV)butoxide (TBO) were obtained from Acros for polyester synthesis. Dibutyltin dilaurate (DBD, 97 %), polycaprolactone diol (PCL, average
Mn=530 g/mol and 1250 g/mol) and 2,2,2‐trifluoroethanol (TFE, 99 %) were purchased from Aldrich. 1,1,1,3,3,3‐Hexafluoro‐2‐propanol (HFIP, 99 %), tetrahydrofuran (THF) and diethyl ether were obtained from Biosolve. Deuterated chloroform (CDCl3, 99 %) was purchased from Cambridge Isotope Laboratory, Inc. (CIL). Lipase from Aspergillus niger
(184 U/g) was obtained from Sigma. A commercial grade PA6 (Akulon, Mn=31 kg/mol, PDI=2.0) was provided by DSM and was used as a reference for biodegradation analysis. All chemicals were used as received, unless otherwise mentioned.
2.2.2 Synthesis of diamine end‐capped PA6
For the synthesis of diamine end‐capped PA6 a batch reactor with a capacity of 380 mL was used. Temperature and pressure were controlled via a computer. First, 100 g (0.88 mol) CL was charged to the reactor and heated until complete melting. Later, 3, 6 or 8 g p‐ XDA (0.022, 0.044, 0.059 mol, respectively) and 3 g (0.17 mol) water were added. The polymerizations were carried out at 250 °C at 3 bar for 6 hours under the flow of N2 gas and with continuous mechanical stirring. Samples for SEC analysis were withdrawn at various time intervals. Final products were extracted with water at 80 °C for 20 hours, filtered under vacuum and dried in an oven at 80 °C for at least 24 hours. Samples were investigated by using SEC, NMR, DSC and titration analysis.
2.2.3 Synthesis of hydroxyl end‐capped oligoester
For the synthesis of hydroxyl end‐capped oligoesters 3.5 g (46.4 mmol) or 4.0 g (52.2 mmol) 1,3‐propane diol (PD) and 5 g (29 mmol) dimethyl adipate (DMA) were put in a 100 mL three neck flask. All the reactions were performed under argon with strong agitation at 180 °C in the melt using 20 mg TBO catalyst. Temperature control was provided by a heating mantle connected to a temperature controller. The reactor was equipped with a distillation set up to remove the methanol that was produced during the polymerization. The reaction time varied between 2.5‐3 hours. After cooling of the polymer to room temperature, it was put in methanol, precipitated by immersing in a liquid N2 and acetone mixture and then filtered. In every case, these steps were carried out three times for the complete removal of the excess diol and the catalyst and later followed by drying in a rotary evaporator and a vacuum oven. The polymers were investigated by using SEC and NMR.
2.2.4 Synthesis of diisocyanate end‐capped polycaprolactone
5.6 g (32 mmol) TDI was placed in a Schlenk vessel which was connected to argon. 10 g (8 mmol) PCL was dissolved in 20 ml THF and placed in an addition funnel. After the addition of 1 drop of dibutyltin dilaurate (DBD), the Schlenk flask was heated to 65 °C and the slow addition of PCL solution to TDI was started with a rate of 1 drop/2 sec under strong agitation. Heating and stirring were stopped after 2 hours. The product was slowly added into diethyl ether which was cooled in an acetone‐liquid N2 mixture, which resulted in precipitation of the polymer. The solvent was removed from the polymer‐diethyl ether mixture to another flask by using a filtrating cannula and a filter by applying a pressure difference. In every case these steps were carried out three times to assure complete removal of excess diisocyanate and the catalyst. Later, residual solvent was removed by using reduced pressure. The product was characterized by NMR and titration.
2.2.5 Copolymer synthesis
Totally dry 10 g (4 mmol) diamine end‐capped PA6 and 5 g (4 mmol) diisocyanate end‐ capped polyester were put in a 100 mL three neck round bottom flask under argon and dissolved in 50 ml HFIP for mixing on the molecular level. After complete dissolution, HFIP was slowly removed by vacuum distillation. This was done at room temperature to avoid the reaction between isocyanate end groups of the PCL and hydroxyl groups of HFIP. Then, the lump of material was taken out of the flask, ground in liquid N2, sieved and reduced pressure was applied again. As soon as the particles were almost totally dry, the product was stirred and heated gradually up to 160 °C, which is below the melting temperature of the polyamide. Reaction was continued overnight. Polymer fractions were investigated via SEC, FTIR and DSC.
2.2.6 Enzymatic and non‐enzymatic hydrolysis
Biodegradation studies were performed with and without enzyme. For both methods, polymer films (25‐30 mg) with an average thickness of 0.4 mm prepared by solvent casting
in HFIP were incubated in separate tubes filled with 10 mL phosphate buffer solution (pH 7.5) which were kept at 25±1 °C. The reference PA6 film was prepared by compression molding. For the enzymatic degradation, lipase from Aspergillus niger (1.6 U/mL) was used and the media was replaced periodically. Films were removed from the media at specific time intervals, washed with distilled water, dried and weighed to determine the weight loss. The morphology of the films was investigated by SEM.
2.2.7 Characterization
2.2.7.1 Size Exclusion Chromatography (SEC)
Size exclusion chromatography (SEC) was used to determine molecular weights and molecular weight distributions, Mw/Mn, of polymer samples. For the PA6 samples and for the blocky polyesteramides SEC in HFIP was performed on a system equipped with a Waters 1515 Isocratic HPLC pump, a Waters 2414 refractive index detector (35 °C), a Waters 2707 autosampler, and a PSS PFG guard column followed by 2 PFG‐linear‐XL (7 µm, 8*300 mm) columns in series at 40 °C. HFIP with potassium trifluoroacetate (3 g/L) was used as eluent at a flow rate of 0.8 mL/min. The molecular weights were calculated against poly(methyl methacrylate) standards (Polymer Laboratories, Mp = 1020 g/mol up to Mp = 1.9*106 g/mol). For the polyester samples SEC in THF was performed on a Waters Alliance system equipped with a Waters 2695 separation module, a Waters 2414 refractive index detector (40 °C), a Waters 2487 dual absorbance detector, and a PSS SDV 5 μ guard column followed by 2 PSS SDV linear XL columns in series of 5 μ (8*300) at 40 °C. THF, stabilized with 2,6‐di‐tert‐butyl‐4‐methylphenol (BHT), was used as eluent at a flow rate of 1 mL/min. The molecular weights were calculated with respect to polystyrene standards (Polymer Laboratories, Mp = 580 Da up to Mp = 7.1*106 Da). Before SEC analysis was performed, the samples were filtered through a 0.2 µm PTFE filter (13 mm, PP housing, Alltech).
2.2.7.2 Nuclear Magnetic Resonance Spectroscopy (NMR)
1H NMR spectra of the polymers were recorded on a Varian 400 MHz spectrometer at 25 °C. PA6 containing samples were dissolved in a 2:1 vol% CDCl3:TFE mixture, whereas the analyses of PCL and its derivatives were performed in CDCl3. For the PA6 polymers, the number average molecular weight Mn was calculated from the NMR spectra by estimating the ratio of the integrals of the proton signals of repeat units to the corresponding end groups.
2.2.7.3 Differential Scanning Calorimetry (DSC)
Melting (Tm) and crystallization temperatures (Tc) as well as enthalpies of melting (∆Hm) and crystallization (∆Hc) of the polymers were measured using a TA Instruments Q100 calorimeter. For all the measurements 4‐6 mg samples and a heating rate of 10°C min–1 were used. DSC measurements of fully amine end‐capped PA6 polymers were carried out from 0°C to 260°C whereas the rest of the samples were analyzed from ‐50°C to 220°C. For each measurement the second heating curve was used to determine the Tm. For the determination of both Tm and Tc peak maximums were taken into account.
2.2.7.4 Fourier Transform Infrared Spectroscopy (FTIR)
The presence of various chemical linkages of the products was derived from FTIR‐ATR spectra that were obtained on a Bio‐Rad Excalibur FTS3000MX spectrophotometer. The measurements were performed by making 50 scans using a golden gate set‐up, equipped with a diamond ATR crystal. The Varian Resolution Pro software version 4.0.5.009 was used for the analysis of the spectra.
2.2.7.5 Potentiometric titration
For the determination of amine [NH2] and carboxylic acid [COOH] end group content, potentiometric end group titrations were done at room temperature in non‐aqueous environment using phenolic solvents. Molecular weight of the polyamides were calculated by using the formula 2*106/([NH2]+[COOH]). Isocyanate end‐group titration was done by
using the back titration method. The sample was dissolved in THF and then, mixed with 10 mL 2.0 M diisobutylamine solution and finally titrated with 1.0 M HCl solution in IPA. Both blank and sample measurements were repeated at least three times. Molecular weight of the polyester was calculated by using the formula MWKOH*2*103/[OH]. 2.2.7.6 Scanning Electron Microscopy (SEM) Surface changes of the polymer films after degradation were observed by using Quanta 3D FEG (FEI) scanning electron microscopy (SEM) equipped with a field emission electron source. High vacuum conditions were applied and a secondary electron detector was used for image acquisition. No additional sample treatment, such as surface etching or coating with a conductive layer, has been applied before surface scanning. Standard acquisition conditions for charge contrast imaging were used.
2.3 Results and Discussion
Synthesis of polyamide 6‐polycaprolactone (PA6‐PCL) or polyamide 6‐polypropylene adipate (PA6‐PPA) block copolymers consisted of three synthetic steps as presented in Figure 2.1. Firstly, low molecular weight fully diamine end‐capped PA6 was synthesized. Later, fully diisocyanate end‐capped PPA and PCL oligoester was synthesized and finally solution and solid‐state polymerization was performed with the co‐reactive oligoester and PA6 components. For the diisocyanate end‐capped oligoester synthesis initially hydroxyl end‐capped polypropylene adipate was synthesized and later end‐capping with toluene diisocyanate was done. Since this polyester had poor properties at room temperature, later fully hydroxyl end‐capped polycaprolactone was used which is commercially available.
