Thermodynamics of Phase Behavior in PEO/P(EO‐b‐DMS) Homopolymer and Block Co‐Oligomer Mixtures under Pressure

The cloud‐point temperatures (T_cl's) of poly(ethylene oxide) (PEO) and poly(ethylene oxide)‐block‐polydimethylsiloxane (P(EO‐b‐DMS)) homopolymer and block‐oligomer mixtures were determined by turbidity measurements over a range of temperatures (105 to 130 °C), pressures (1 to 800 bar), and compositions (10–40 wt.‐% PEO). The system phase separates upon cooling and T_cl was found to decrease with an increase in pressure for a constant composition. In the absence of special effects, this finding indicates negative excess volumes. Special attention was paid to the demixing temperatures as a function of the pressure for the different polymer mixtures and the plots in the T‐? plane (where ? signifies volume fractions). The cloud‐point curves of the polymer mixture under pressures were observed for different compositions. The Sanchez‐Lacombe (SL) lattice fluid theory was used to calculate the spinodals, the binodals, the Flory‐Huggins (FH) interaction parameter, the enthalpy of mixing, and the volume changes of mixing. The calculated results show that modified P(EO‐b‐DMS) scaling parameters with the new combining rules can describe the thermodynamics of the PEO/P(EO‐b‐DMS) system well with the SL theory.

Cloud point curves for various PEO/P(EO‐b‐DMS) polymer mixtures at various pressures on the T‐?_PEO plane.     

Shear Induced Demixing and Rheological Behavior of Aqueous Solutions of Poly(N‐isopropylacrylamide)

The interrelation between the phase separation behavior and the rheological performance of aqueous solutions of high molecular weight (M _w = 1 600 kg/mol) poly(N‐isopropylacrylamide) was investigated. The system demixes upon heating and the cloud point temperature, T_cp decreases steadily with rising polymer concentration up to 10 wt.‐%. The application of shear supports phase separation and reduces T_cp markedly. This observation is interpreted in terms of destruction of intersegmental clusters formed in the quiescent state owing to favorable interactions. Intrinsic viscosities and Huggins coefficients as well as the viscosities, η at higher polymer concentrations are closely connected with the thermodynamic conditions. [η] decreases by almost two orders of magnitude upon heating, whereas the corresponding increase of k_H is less pronounced. The η values (constant shear rate) of the moderately concentrated solutions as function of T pass a maximum at the corresponding phase separation temperatures. The existence of clusters also manifests in terms of stress overshoot and of particularities observed with solutions that are sheared for the first time.

Transmittance (ratio of intensities of the transmitted light and the incidence light) of a 2.0 wt.‐% PNIPAm solution in water as a function of temperature at different shear rates indicated. Heating rate is 0.22 °C/min.     

Thermoresponsive Properties of Poly(N‐vinylcaprolactam)‐Poly(ethylene oxide) Aqueous Systems: Solutions and Block Copolymer Networks

Two‐ and three‐dimensional phase diagrams have been constructed for thermosensitive poly(N‐vinylcaprolactam)‐poly(ethylene oxide) (PVCL‐PEO) aqueous systems. Both solutions and swollen block copolymer networks have been investigated to elucidate the effect of the copolymer content and crosslinking density on their temperatures of phase separation (T_ph.s.). The introduction of hydrophilic PEO into an aqueous solution of PVCL decreases its T_ph.s.. This suggests that the strength of the hydrogen bonds within the thermoresponsive PVCL‐water system is weakened by the introduction of PEO that also interacts with water. Based on the DSC investigation of the swollen networks, it was found that the influence of PEO on the phase behavior of weakly crosslinked networks is comparable with that of PVCL‐PEO‐H₂O solutions. For networks with a higher degree of crosslinking, the presence of the crosslinks is of major importance for the explanation of the T_ph.s. location. This detailed phase analysis led to the proposal of an irregular water distribution in these swollen networks.     

Synthesis and Hydrolysis of α,ω‐Perfluoroalkyl‐Functionalized Derivatives of Poly(ethylene oxide)

Summary: Two series of novel α,ω‐perfluoroalkyl terminated esters of poly(ethylene oxide) (PEO) (R_F‐PEO) having the general structure C_mF_2m+1? COO? (CH₂? CH₂? O)_n? OC? C_mF_2m+1, with m = 1,2,3,4 or 5 have been synthesized. The influences of the PEO molar mass, the length of the perfluoroalkyl group (R_F) and temperature on the cleavage of the ester bridge in aqueous solution and the effect of the hydrolysis process on the size of aggregates formed in water were studied. According to ¹H and ¹⁹F NMR measurements the degree of functionalization obtained (up to 96 mol‐%) increases with the decrease in the length of the R_F group. All of the derivatives showed ester cleavage in water in short time scales. The rates of hydrolysis of the ester bridge in aqueous solution were determined from pH‐measurements. It was verified that the rate law for hydrolysis corresponds to a pseudo‐first order type. The hydrolysis kinetic constant k increased with a decrease in the length of the R_F group ranging from 0.2 × 10^?3 s^?1 for the longest R_F group (C₅F₁₁? ) up to 1.2 × 10^?2 s^?1 for the shortest R_F group (CF₃? ). The value of k depended almost exclusively on the length of the perfluoroalkyl chain and was independent of the length of the PEO backbone (1 000 or 2 000 g · mol^?1), as long as no additional phenomenon such as phase separation was present. It was also found that the change in the value of k with temperature followed a non‐Arrhenius pattern and there was an evident relationship between the non‐linearity in the ln k vs. 1/T relation with increasing temperatures and the occurrence of a macroscopic phase separation of LCST type. Dynamic light scattering measurements showed the coexistence of unimers with associated species with apparent hydrodynamic radii (R_h) of approximately 20–45 nm for all samples in aqueous solutions. These species might correspond to aggregates of a few micelles. For some samples also larger aggregates were found with R_h in the 100–500 nm range, which might be attributed to clusters of micelles.

     

Synthesis and Characterization of Amphiphilic Poly(ethylene oxide)‐block‐poly(hexyl methacrylate) Copolymers

We describe the preparation of amphiphilic diblock copolymers made of poly(ethylene oxide) (PEO) and poly(hexyl methacrylate) (PHMA) synthesized by anionic polymerization of ethylene oxide and subsequent atom transfer radical polymerization (ATRP) of hexyl methacrylate (HMA). The first block, PEO, is prepared by anionic polymerization of ethylene oxide in tetrahydrofuran. End capping is achieved by treatment of living PEO chain ends with 2‐bromoisobutyryl bromide to yield a macroinitiator for ATRP. The second block is added by polymerization of HMA, using the PEO macroinitiator in the presence of dibromobis(triphenylphosphine) nickel(II), NiBr₂(PPh₃)₂, as the catalyst. Kinetics studies reveal absence of termination consistent with controlled polymerization of HMA. GPC data show low polydispersities of the corresponding diblock copolymers. The microdomain structure of selected PEO‐block‐PHMA block copolymers is investigated by small angle X‐ray scattering experiments, revealing behavior expected from known diblock copolymer phase diagrams.

SAXS diffractograms of PEO‐block‐PHMA diblock copolymers with 16, 44, 68 wt.‐% PEO showing spherical (A), cylindrical (B), and lamellae (C) morphologies, respectively.     

Rheological Investigation of Shear Induced‐Mixing and Shear Induced‐Demixing for Polystyrene/Poly(vinyl methyl ether) Blend

Full Paper: The phase behavior of polystyrene (PS) and poly(vinyl methyl ether) (PVME) blend has been investigated rheologically as a function of temperature, composition and oscillating shear rate as well as different heating rates. An LCST (lower critical solution temperature)‐type phase diagram was detected rheologically from the sudden changes in the slopes of the dynamic temperature ramps of G′ at given heating and shear rate values. The rheological cloud points were dependent on the heating rate, , and oscillating shear rate, . The cloud points shifted a few degrees to higher temperatures with increasing and reached an equilibrium value (heating rate independent) at °C/min. The phase diagrams of the blends detected at = 0.1 and 1 rad/s were located in lower temperature ranges than the quiescent phase diagram, i.e., oscillating shear rate induced‐demixing at these two values for the shear rate. On the other hand, at = 10 rad/s, the phase diagram shifted to higher temperatures, higher than the corresponding values found under quiescent conditions, i.e., shear induced‐mixing took place. Based on these two observations, shear induced‐demixing and shear induced‐mixing can be detected rheologically within a single composition at low and high shear rate values, respectively, and this is in good agreement with the previous investigation using simple shear flow techniques. In addition, the William, Landel and Ferry (WLF)‐superposition principle was found to be applicable only in the single‐phase regime; however, the principle broke‐down at a temperature higher than or equal to the cloud point. Furthermore, different spinodal phase diagrams were estimated at different oscillating shear rates based on the theoretical approach of Ajji and Choplin.

Spinodal phase diagrams at different oscillating shear rates.     

Janus Micelle Formation Induced by Protonation/Deprotonation of Poly(2‐vinylpyridine)‐block‐Poly(ethylene oxide) Diblock Copolymers

A novel approach to amphiphilic polymeric Janus micelles based on the protonation/deprotonation process of poly(2‐vinylpyridine)‐block‐poly(ethylene oxide) (P2VP‐b‐PEO) diblock copolymers in THF is presented. It is found that addition of HCl to the micelles solution of P2VP‐b‐PEO copolymers leads to the formation of vesicles. Subsequently mixing a small amount of hydrazine monohydrate with the vesicle solution can induce the dissociation and reorganization of the vesicles into Janus micelles. When HCl is replaced by HAuCl₄ precursors, composite Janus particles containing gold in P2VP blocks are obtained.

     

Association Behavior between End‐Functionalized Block Copolymers PEO‐PPO‐PEO and Poly(acrylic acid)

Summary: The end groups of ABA‐triblock copolymers HO–PEO–PPO–PEO–OH, (PEO – poly(ethylene oxide), PPO – poly(propylene oxide)), have been modified with ammonia, ethylene diamine and linear polyethylenimine (LPEI) by substitution of the α,ω‐ditosyl ester of the triblock copolymer (TsO–PEO–PPO–PEO–OTs) with amines, or by the hydrolysis of the corresponding poly(2‐methyl‐2‐oxazoline) (PMeOx) containing ABCBA block copolymers. The latter block copolymer structures have been obtained by the polymerization of MeOx using TsO–PEO–PPO–PEO–OTs as a macro‐initiator. Adding poly(acrylic acid) (PAA) to these (poly)amine terminated block copolymers leads to the formation of networks through a combination of PAA–PEO hydrogen bonding and PAA–(poly)amine acid‐base reaction. Depending on the number of amino groups at both chain ends of the block copolymer, the corresponding complexes behave as liquids, gels or precipitates. Introduction of as little as 1–5 wt.‐% block copolymers H₂N–PEO–PPO–PEO–NH₂ or H₂NCH₂CH₂NH–PEO–PPO–PEO–NHCH₂CH₂NH₂ to the system of HO–PEO–PPO–PEO–OH/PAA leads to viscous liquids with strong shear‐thickening behavior.

Reversible gel formation via the ternary PAA/HO–PEO–PPO–PEO–OH/H₂N–PEO–PPO–PEO–NH₂ system under shear conditions.     

Quantitative Analysis of Compositional Distribution in Biodegradable Poly(ethylene oxide)/Poly(3‐hydroxybutyrate) Blend Film with Compositional Gradient by FT‐IR Microspectroscopy

Summary: FT‐IR microspectroscopy was used to map the compositional distribution along the cross‐section on the gradient films of fully biodegradable poly(ethylene oxide) (PEO)/poly(3‐hydroxybutyrate) (PHB) blend system. First, a linear fitting line for a calibration, related the absorbance ratio of two peaks to the fraction of PEO in the blend, was established. During linear fitting, a new equation was deduced and the influence of the different crystallinities on the absorption peaks at the wavenumber 962 and 1 342 cm^?1 for PEO, 980 and 1 380 cm^?1 for PHB, have been taken into account. For the PEO/PHB blend system, it was found that the crystallinity has little effect on the absorbance ratios of A₉₆₂/A_{1 380} and A_{1 342}/A_{1 380}. Based on the results from the linear fitting, which comes from the relation of the absorbance ratios of A₉₆₂/A_{1 380} or A_{1 342}/A_{1 380} and the combination of the weight fraction of PEO (W_PEO/(1 ? W_PEO)), the compositional distributions on the cross‐section of three kinds of compositional gradient films of the PEO/PHB blend prepared by different technologies have been successfully estimated. Furthermore, the better method for preparing the PEO/PHB blend film with compositional gradient was found based on the result of the quantitative analysis of the compositional distribution.

The compositional distribution along the cross‐section of the gradient films based on PEO/PHB binary system for the Type I gradient film.     

Thermoresponsive UCST‐Type Behavior of Interpolymer Complexes of Poly(ethylene glycol) and Poly(poly(ethylene glycol) methacrylate) Brushes with Poly(acrylic acid) in Isopropanol

Upper critical solution temperature (UCST)‐type thermoresponsive behavior of poly(ethylene glycol)–poly(acrylic acid) (PEG–PAA) and poly(poly(ethylene glycol) methacrylate)–poly(acrylic acid) (PPEGMA–PAA) interpolymer complexes has been observed in isopropanol. For these investigations, PPEGMA and PAA with various average molecular weights have been synthesized by atom transfer radical polymerization. It has been found that both the PEG and PPEGMA have lower cloud point temperatures (T _cp) than its mixed polymer solutions with PAA, whereas PAA does not show such behavior in the investigated temperature range. These findings indicate the reversible formation of interpolymer complexes with variable structure and composition in the solutions of the polymer mixtures in isopropanol. Increasing the ethylene glycol/acrylic acid molar ratio or the molecular weight of either the PAA or the H‐acceptor PEG component of the interpolymer complexes increases the UCST‐type cloud point temperatures of these interpolymer systems. The polymer–polymer interactions by hydrogen bonds between PAA and PEG or PPEGMA and the correlations between T _cp and structural parameters of the components revealed in the course of these investigations may be utilized for exploring well‐defined UCST‐type material systems for various applications.

     

Molecular Motions in the Supramolecular Complexes between Poly(ε‐caprolactone)‐Poly(ethylene oxide)‐Poly(ε‐caprolactone) and α‐ and γ‐Cyclodextrins

The structure and molecular motions of the triblock copolymer PCL‐PEO‐PCL and its inclusion complexes with α‐ and γ‐cyclodextrins (α‐ and γ‐CDs) have been studied by solid‐state NMR. Different cross‐polarization dynamics have been observed for the guest polymer and host CDs. Guest–host magnetization exchange has been observed by proton spin lattice relaxation T₁, proton spin lattice frame relaxation T_1ρ and 2D heteronuclear correlation experiments. A homogeneous phase has been observed for these complexes. Conventional relaxation experiments and 2D wide‐line separation NMR with windowless isotropic mixing have been used to measure the chain dynamics. The results show that for localized molecular motion in the megahertz regime, the included PCL block chains are much more mobile than the crystalline PCL blocks in the bulk triblock copolymer. However, the mobility of the included PEO block chains is not very different from the amorphous PEO blocks of the bulk sample. The cooperative, long chain motions in the mid‐kilohertz regime for pairs of PCL‐PEO‐PCL chains in their γ‐CD channels seem more restricted than for the single PCL‐PEO‐PCL chains in the α‐CD channels, however, they are not influencing the more localized, higher frequency megahertz motions.     

Miscibility Enhancement on the Immiscible Binary Blend of Poly(vinyl phenol) and Poly(acetoxystyrene) with Poly(ethylene oxide)

Summary: The miscibility and hydrogen‐bonding behaviors of ternary polymer blends of poly(ethylene oxide) (PEO)/poly(vinyl phenol) (PVPh)/poly(acetoxystyrene) (PAS) were investigated by using DSC and Fourier transform infrared spectroscopy (FTIR). The PEO is miscible with both PVPh and PAS based on the observed single T_g over the entire composition range. FTIR was used to study the hydrogen‐bonding interactions between PEO with PAS and PVPh, respectively. Quantitative analyses show that the strength of hydrogen‐bonding strength is of the order of the hydroxyl‐ether inter‐association of PVPh/PEO blend > the hydroxyl‐hydroxyl self‐association of pure PVPh > the hydroxyl‐carbonyl inter‐association PVPh/PAS blend at room temperature. Furthermore, the addition of PEO is able to enhance the miscibility of immiscible PVPh/PAS binary blends at lower (20 wt.‐%) or higher (60 and 80 wt.‐%) PEO content. However, there exists a closed immiscibility loop in the phase diagram at 40 wt.‐% PEO content due to the “Δχ” and “ΔK” effects in this hydrogen‐bonded ternary polymer system. Therefore, an interesting and unusual sandwich phase diagram has been observed in this ternary polymer blend.

Ternary phase diagram of the PEO/PAS/PVPh system.     

Influence of shear on the demixing of polymer solutions, 1 apparatus and experimental results

An apparatus for turbidimetric measurements of demixing temperatures under shear flow is presented, and the results of experiments with trans-decahydronaphthalene/polystyrene (TD/PS) solutions, investigating molecular weights ranging from 100 kg/mol to 1770 kg/mol, are compared with viscometric data. It is found that the sign and magnitude of shear effects depend on molecular weight (M_w), polymer concentration (c₂), and shear rate (). For the first time, it was possible to study a solution which exhibits shear dissolution at low shear rates but shear demixing at high shear rates.     

Spinodal Decomposition in Binary Blend of Poly(methyl methacrylate)/Poly(α‐methyl styrene‐co‐acrylonitrile): Analysis of Early and Late Stage Demixing

Summary: The phase separation kinetics of a poly(methyl methacrylate), PMMA, and poly(α‐methylstyrene‐co‐acrylonitrile), PαMSAN, blend exhibiting a LCST‐type phase diagram have been investigated as functions of temperature and demixing time for the near critical composition (PαMSAN/PMMA = 25:75) using a time‐resolved light scattering technique. We found that the scattering data in the early stage spinodal decomposition (SD) can be well described by the linearized Cahn–Hilliard theory. Spinodal temperature T_s ～ 171 °C was determined from D_app versus T and qequation/tex2gif-stack-1.gif versus T based on the analysis of Cahn theory in the early stage SD, where D_app is the apparent diffusion coefficient and q_m is the scattering vector at the maximum scattering intensity. The value of T_s obtained from the analysis of light scattering data was in good agreement with the phase diagram obtained visually at the equilibrium condition. The LCST‐type phase diagram of this blend was also calculated using the Flory–Huggen theory. The estimated interaction parameter used for the calculation of the phase diagram was found to be composition and temperature dependent. The coarsening behavior of the blend at the late stage SD was also studied by analyzing the magnitudes of q_m and I_m at various demixing times and temperatures based on the nonlinear statistical theories. Both of q_m and I_m obtained at different times and temperatures can be superposed and reduced to the respective master curves by horizontal shifting when they are plotted against log (t/a_T) at a given reference temperature, where a_T is the temperature‐dependent shift factor. The shape of the phase separation domain structures as a function of time during the SD was also considered. The scaling structure function F(x,t;T) = q_m(t;T)³I(q,t;T) versus q/q_m was found to be time independent and falls onto a universal curve in the late stage SD as a result of the dynamical self‐similarity accompanying with the phase separation process.

Optical microscopy pictures for the structure development of spinodal decomposition for PαMSAN/PMMA = 25:75 blend at 180 °C for different demixing times.     

Thermo‐Responsive and Emulsifying Properties of Poly(N‐vinylcaprolactam) Based Graft Copolymers

Thermo‐responsive graft copolymers have been synthesized based on a poly(N‐vinylcaprolactam) (PVCL) backbone and either hydrophilic poly(ethylene oxide) (PEO) or hydrophobic poly(tetrahydrofuran) (PTHF) side chains. The phase separation behavior of the graft polymers in water was studied by transmittance measurements and compared to that of the corresponding swollen segmented polymer networks and aqueous solutions of both polymers. The influence of the concentration and length of the grafts on the cloud point temperature (T_CP) has been demonstrated. PVCL‐g‐PTHF copolymers have been synthesized by using the macromonomer technique, i.e. the radical copolymerization of VCL with a PTHF macromonomer. A special feature of these amphiphilic graft copolymers is their ability to stabilize aqueous emulsions below the T_CP and to suddenly break them above the T_CP. PVCL‐g‐PEO copolymers were prepared by a grafting onto method. First, succinimide groups were introduced in the backbone, to which amino terminated PEO chains were grafted in the second step. This leads to di‐hydrophilic copolymers that become amphiphilic after heating their aqueous solutions above the T_CP.

     

Influence of shear on the demixing of polymer solutions, 2. Stored energy and theoretical calculations

Phase diagrams were calculated for flowing solutions of polystyrene in trans-decahydronaphthalene assuming that the energy (E_s) stored in the sheared state has to be added to the Gibbs energy of mixing; E_s is computed from viscometric data (flow curves). The resulting phase diagrams exhibit two maxima in most of the cases instead of only one for the stagnant solutions. A comparison with experimental data (Part 1) shows that the calculated demixing behaviour agrees well for moderate shear rates and concentrations of the polymer. Deviations in the semidilute region, particularly for higher values, are a consequence of the fact that so far no reliable theoretical relation has been given for these conditions to calculate E_s from viscometric data. In all cases, however, the present attempt describes the characteristics of the phase diagrams very well, namely the eulytic points and the inversion of shear dissolution into shear demixing.     

Allyl End‐Functionalized Poly(ethylene oxide)‐block‐poly(methylidene malonate 2.1.2) Block Copolymers: Synthesis,Characterization, and Chemical Modification

Summary: Poly(ethylene oxide)‐block‐poly(methylidene malonate 2.1.2) block copolymer (PEO‐b‐PMM 2.1.2) bearing an allyl moiety at the poly(ethylene oxide) chain end was synthesized by sequential anionic polymerization of ethylene oxide (EO) and methylidene malonate 2.1.2 (MM 2.1.2). This allyl functional group was subsequently modified by reaction with thiol‐bearing functional groups to generate carboxyl and amino functionalized biodegradable block copolymers. These end‐group reactions, performed in good yields both in organic media and in aqueous micellar solutions, lead to functionalized PEO‐b‐PMM 2.1.2 copolymers which are of interest for cell targeting purposes.

Synthetic route to α‐allyl functionalized PEO‐b‐PMM 2.1.2 copolymers.     

Crystallization and Ring‐Banded Spherulite Morphology of Poly(ethylene oxide)‐block‐Poly(ε‐caprolactone) Diblock Copolymer

Summary: The crystallization behavior of crystalline‐crystalline diblock copolymer containing poly(ethylene oxide) (PEO) and poly(ε‐caprolactone) (PCL), in which the weight fraction of PCL is 0.815, has been studied via differential scanning calorimeter (DSC), wide‐angle X‐ray diffraction (WAXD), and polarized optical microscopy (POM). DSC and WAXD indicated that both PEO and PCL blocks crystallize in the block copolymer. POM revealed a ring‐banded spherulite morphology for the PEO‐b‐PCL diblock copolymer.

DSC heating curve for the PEO‐b‐PCL block copolymer.     

pH‐Sensitive Micelles Formed by Interchain Hydrogen Bonding of Poly(methacrylic acid)‐block‐Poly(ethylene oxide) Copolymers

pH‐sensitive micelles formed by interchain hydrogen bonding of poly(methacrylic acid)‐block‐poly(ethylene oxide) copolymers were prepared and investigated at pH < 5. Both and R_h of the micelles increase with decreasing pH of the solution, displaying an asymptotic tendency at low pH values. The observed micelles are well‐defined nanoparticles with narrow size distributions (polydispersity ΔR_h/R_h ≤ 0.05) comparable with regular diblock copolymer micelles. The CMCs occur slightly below c = 1 × 10^?4 g · mL^?1. The micelles are negatively charged and their time stability is lower than that of regular copolymer micelles based purely on hydrophobic interactions.

     

Effects of Residual Nuclei on Crystals and Equilibrium Melting Points of Dual Crystals in Poly(ethylene 2,6‐naphthalene‐dicarboxylate)

Summary: The crystal structures and possible transformations in poly(ethylene naphthalate) (PEN) were examined using differential scanning calorimetry (DSC) and wide‐angle X‐ray diffraction (WAXD). The crystal cells (α‐ and β‐form) were dependent on several factors related to their thermal histories. For PEN subjected to T_max = 280 °C (where T_max is the maximum melt temperature at which the samples were treated before crystallization), only the α‐form developed regardless of the temperature of crystallization or cooling rate. However, for PEN subjected to T_max = 300 °C, PEN developed mixed dual crystals, with greater fractions of the α‐form at low temperatures of crystallization (220 °C or lower) or higher cooling rates, but higher β‐form fractions at higher crystallization temperatures (>230 °C). The difference in the relative stability of these two crystals was utilized to prepare PEN containing only the α‐form or only the β‐form. The equilibrium melting temperatures of these two crystal forms in PEN were then measured separately. For PEN containing only the β‐form, T (289.3 °C) was found by using the Hoffman‐Weeks technique, while for PEN containing only the α‐form, T was determined using the same technique but was found to be lower (282.6 °C).

Comparison of Hoffman‐Weeks extrapolations for the α‐crystal and β‐crystal, respectively, in PEN.