Relaxation in Poly(alkyl methacrylate)s: Crossover Region and Nanophase Separation

Relaxations in a poly(alkyl methacrylate) series are systematically influenced by chemical modifications like the variation of side‐chain length, random copolymerization, or molecular weight. Recent results concerning the influence of chemical modifications on special parts of the relaxation chart are reviewed. The discussion is focused on two points: (i) The influence of chemical modifications on the crossover region of dynamic glass transition, where the relaxation times of α relaxation and Johari Goldstein mode β approach each other, is discussed. A general crossover scenario with a separate onset of cooperative α relaxation is observed for all lower members of this series. High temperature process a above and cooperative α relaxation below the crossover are shown to be distinct processes. Chemical modifications related to an increase in free volume shift this scenario mainly to lower frequency and temperature. Further details depend on the specific modification. (ii) The nanophase separation of incompatible main‐ and side‐chain parts in all higher members of the poly(alkyl methacrylate) series is discussed. This effect is concluded from the coexistence of two dynamic glass transitions in these homopolymers, the conventional a (or α) process and an additional low temperature glass transition α_PE . It is shown that the low T_g process is related to cooperative motions in the polyethylene‐like side‐chain parts. The existence of static nanodomains in the range 0.5 to 1.5 nm is confirmed by means of wide and small angle X‐ray scattering data. The estimated nanodomain size is compared with the size of dynamic heterogeneities estimated independently from calorimetric data for the polyethylene‐like glass transition using the fluctuation approach.     

Revisiting the crystallization of poly(2‐alkyl‐2‐oxazoline)s

Poly(2‐alkyl‐2‐oxazoline)s (PAOx) exhibit different crystallization behavior depending on the length of the alkyl side chain. PAOx having methyl, ethyl, or propyl side chains do not show any bulk crystallization. Crystallization in the heating cycle, that is, cold crystallization, is observed for PAOx with butyl and pentyl side chains. For PAOx with longer alkyl side chains crystallization occurs in the cooling cycle. The different crystallization behavior is attributed to the different polymer chain mobility in line with the glass transition temperature (T_g) dependency on alkyl side chain length. The decrease in chain mobility with decreasing alkyl side chain length hinders the relaxation of the polymer backbone to the thermodynamic equilibrium crystalline structure. Double melting behavior is observed for PButOx and PiPropOx which is explained by the melt‐recrystallization mechanism. Isothermal crystallization experiments of PButOx between 60 and 90 °C and PiPropOx between 90 and 150 °C show that PAOx can crystallize in bulk when enough time is given. The decrease of T_g and the corresponding increase in chain mobility at T > T_g with increasing alkyl side chain length can be attributed to an increasing distance between the polymer backbones and thus decreasing average strength of amide dipole interactions. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 721–729     

Order in amorphous di‐n‐alkyl itaconate polymers,copolymers, and blends

The presence of a main‐chain correlation distance (d_II) in the poly(di‐n‐alkyl itaconate)s was confirmed with small‐angle X‐ray scattering/wide‐angle X‐ray scattering measurements taken over the temperature range of 293–478 K. Data for a series of alkyl acrylate polymers were also obtained for comparison. The intensity of the itaconate d_II peak was significant and indicated a greater level of nanophase formation than in analogous systems. In the lower members of the series, nanophase formation appeared to be further enhanced in the temperature range above the glass‐transition temperature (T_g). This was ascribed to the rapidly increasing main‐chain mobility in this region. Macroscopically phase‐separated itaconate blends displayed the individual d_II nanospacings of each homopolymer component. Copolymers, on the other hand, showed more interesting behavior. Poly(methyl‐co‐di‐n‐butyl itaconate) followed an average behavior in which the d_II spacing and T_g changed progressively with the comonomer content. In contrast, the side‐chain pairing in poly(methyl‐co‐di‐n‐octyl itaconate) generated d_II spacings characteristic of separate methyl and octyl nanodomains. The observation of the dioctyl nanodomains, along with the dioctyl side‐chain lower T_g relaxation event, confirmed the concept of independent side‐chain‐domain relaxation in these polymers. The temperature behavior of the poly(methyl‐co‐di‐n‐octyl itaconate) small‐angle X‐ray scattering profiles and scattering correlation lengths indicated that the two nanodomains were not completely structurally independent. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4000–4016, 2004     

PVT of amorphous and crystalline polymers and their nanocomposites

The Pressure-Volume-Temperature (PVT) of polystyrene (PS), polyamide-6 (PA-6) and their clay-containing polymeric nanocomposites (CPNC) were determined at T = 300-600 K and P = 0.1-190 MPa, thus in the molten, glassy and semicrystalline phase. The melt and glass behavior was interpreted following the Simha-Somcynsky (S-S) cell-hole free volume theory while that of the semicrystalline phase using S-S and the Midha-Nanda-Simha-Jain (MNSJ) cell theory describing crystalline quantum interactions. The theoretical analysis yielded two sets of the interaction parameters, one from the S-S and the other from the MNSJ model. The derivative properties: the compressibility, κ, and thermal expansion coefficient, α, were computed as functions of T, P and clay content, w. These functions, crossing several transition regions, were significantly different for the amorphous PS than for the semicrystalline PA-6. The isobaric PS plots of κ and α vs. T detected secondary transitions at T_β/T_g ≈ 0.9 ± 0.1 and at T_c/T_g = 1.2 ± 0.1. Addition of clay severely affected the vitreous phase (physical aging). In PA-6 systems the behavior was distinctly different than in PS, viz. κ = κ(T) followed a similar function across the melting zone, while α = α(T) dependencies were dramatically different for the solid and molten phase. The theoretical functions in reduced variables provided good basis for explanation of the observed dependencies.     

Poly-1-oxy-2-phenyltrimethylene as studied by 1H pulsed low resolution NMR: a possible oxygen scavenger

Aromatic polymers can adsorb a large amount of oxygen on their aromatic rings. In semicrystalline aromatic polymers, the amount of adsorbed oxygen is much larger in the amorphous phase than in the crystalline counterpart. As a consequence, fully amorphous aromatic polymers are more suitable as oxygen scavengers than semicrystalline polymers.A pulsed low resolution ¹H-NMR relaxation study on semicrystalline and fully amorphous poly-1-oxy-2-phenyltrimethylene (St–CO), deuteriated and not deuteriated on the backbone, is reported. T₁ relaxation values were measured at 30 and 57 MHz and compared with the values of two aromatic polymers previously studied, syndiotactic polystyrene (s-PS) and polyphenyleneoxide (PPO), both patented as oxygen scavengers. For all these polymers, using a set of equations, at each temperature, the amount of adsorbed oxygen was calculated.Very short T₁ values are observed at the low temperature point of PPO which is the best oxygen scavenger at low temperature, while St–CO adsorbs oxygen efficiently at room temperature. Thus, St–CO might be suitable to be used as an oxygen scavenger.In St–CO, a ¹H T_1ρ relaxation study on the rotating frame has also been performed. In the atactic copolymer, in the temperature range 150–160 K, a sharp transition was observed only in the presence of oxygen. In agreement with a previously given interpretation for analogous data, the observed transition might be related to low frequency motions present in low molecular weight components.     

Elevation of the glass transition temperature in flexible‐chain semicrystalline polymers

In the idealized two‐phase model of a semicrystalline polymer, the amorphous intercrystalline layers are considered to have the same properties as the fully‐amorphous polymer. In reality, these thin intercrystalline layers can be substantially influenced by the presence of the crystals, as individual polymer molecules traverse both crystalline and amorphous phases. In polymers with rigid backbone units, such as poly(etheretherketone), PEEK, previous work has shown this coupling to be particularly severe; the glass transition temperature (T_g) can be elevated by tens of degrees celsius, with the magnitude of the elevation correlating directly with the thinness of the amorphous layer. However, this connection has not been explored for flexible‐chain polymers, such as those formed from vinyl‐type monomers. Here, we examine T_g in both isotactic polystyrene (iPS) and syndiotactic polystyrene (sPS), crystallized under conditions that produce a range of amorphous layer thicknesses. T_g is indeed shown to be elevated relative to fully‐amorphous iPS and sPS, by an amount that correlates with the thinness of the amorphous layer; the magnitude of the effect is severalfold less than that in PEEK, consistent with the minimum lengths of polymer chain required to make a fold in the different cases. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1198–1204, 2007     

Well-defined crosslinked materials via ring opening metathesis polymerisation

The work reported here is part of our ongoing programme of work directed. towards the synthesis and characterisation of polymeric materials via ROMP-RIM and ROMP-RTM. It describes the synthesis and characterisation of well-defined linear and crosslinked polymeric materials via ROMP-RTM. The process involves in-mould polymerisation of monofunctional imidonorbornene monomers, with different alkyl side chain lengths, to give a range of linear polymers. The process also involves in-mould copolymerisation of monofunctional imidonorbornene monomers, with different alkyl side chain lengths and difunctional monomers with different alkylene spacer lengths, to produce well-defined crosslinked polymers. The glass transition temperature (T_g) of the linear polymers was found to depend on the length of the alkyl side chain. For the crosslinked materials the results show that as the percentage of the difunctional, crosslinking unit, is increased (1, 5 and 10 molar percentage of the difunctional monomer) the glass transition shifts to a higher temperature, the height of the tanδ peak decreases and the plateau shear modulus above T_g increases. These results are as expected for an increase in the crosslink density of a polymer.     

Thermally stimulated current and DSC studies of the broadened glass transition in liquid crystalline polymers

The high sensitivity of the thermally stimulated current, thermal sampling (TS) method is emphasized in a study of the breadth of the glass transition in several liquid-crystalline polymers (LCPs). Differential scanning calorimetry (DSC) was performed on all samples to further quantify the glass transition regions. For “random” copolyester LCPs with widely varying degrees of crystallinity, including highly amorphous samples, very broad glass tran-sition regions were observed. One semicrystalline alternating copolyester and a series of semicrystalline azomethine LCPs were studied as examples of structurally regular polymers. These exhibited relatively sharp glass transitions more comparable to ordinary isotropic amorphous or semicrystalline polymers. The broad glass transitions in the random copolyesters are attributed to structural heterogeneity of the chains. In one example of a moderate-crystallinity random copolyester LCP (Vectra), glass transitions ranging up to ca. 150°C in breadth were determined by the thermal sampling (TS) method and DSC. In other lower crystallinity copolyester LCPs, the main glass transition temperature as determined by DSC was comparable to that determined by TSC although cooperative relaxations of a minor fraction of the overall relaxing species were detected well below the main T_g, by the TS method and not by DSC. Rapid quenches from the isotropic melt to an isotropic glass were possible with one LCP. The anisotropic and isotropic glassy states for this LCP were found to have the same breadth of the glass transition as was determined by the TS method, although TSC and DSC show that T_g is shifted downward by ca. 15°C in the anisotropic glass as compared to the isotropic glass. © 1993 John Wiley & Sons, Inc.     

Increased glass transition temperature in motionally constrained semicrystalline polymers

A large number of experimental results in the literature support and illuminate a model of behavior of chains and chain segments in the amorphous phase of semicrystalline polymers connecting the elevation of the glass transition temperature (T_g) above its normal value to several kinds of motional restrictions imposed on the chains and parts thereof. Accordingly, polymer chain, chain-segment and chain-fragment motions of all kinds comprise one or more torsions around main-chain bonds from one stable conformation to another, known as rotational isomerizations. When impediments are placed in front of thermal fluctuations and larger transversal and longitudinal motions of polymer chains, segments and shorter fragments in the amorphous phase, and the motions are thus restricted, the glass transition temperature is elevated relative to that of the same amorphous phase in the bulk under normal conditions. The obstructions may prevent either the onset of rotational isomerizations or of their completion once started. The completion of the torsional isomerizations and larger motions may be prevented by eliminating the free spaces necessary to accommodate the volumes of the interconverting chain fragments and segments even when they move in concert, or by preventing the creation of such free spaces. Another way to hinder the completion of such motions is by the introduction into the system of many rigid walls and other interfaces with strong attractive interactions with the polymer, that by geometrical constraints and attractive interactions suppress the rotational and larger motions and prevent their completion. Elimination of the necessary free volume is achievable by the application of compressive pressure, while the introduction of rigid attractive walls may be accomplished by the incorporation of crystallites, as in semicrystalline polymers, or by the addition of rigid finely comminuted foreign additives with very large surface areas or confining voids with high tortuosity. It is believed that motional restrictions imposed on the amorphous phase by the growth faces of polymer crystallites, especially in oriented semicrystalline polymers, are more effective than the restrictions imposed by the fold surfaces of these crystallites. The prevention of the onset of rotational isomerizations and larger motions may be achieved by stretching the polymer chains and chain segments in the amorphous phase and, by one means or another, pinning down the taut chains such that essentially all their rotational isomers are in the trans conformation: they cannot interconvert to the gauche conformation since it requires the chain’s end-to-end distance to decrease. Parallel alignment of relatively taut chain-segments may impose additional geometrical restrictions on both the onset and completion of rotational isomeric torsions and, of course, on longer-range motions. In all cases, the T_g of the motionally constrained parts of the amorphous phase, especially in semicrystalline polymers, is expected to rise. It is likely that the characteristic length associated with transversal motions and their suppression is R_c, the spatial distance between entanglements, which is of the same size scale, and may be the same as the tube diameter of the reptation model. Special emphasis was placed in this work on the semicrystalline polymers poly (ϵ-caprolactam) (nylon-6) and poly (ethylene terephthalate) (PET). © 1998 John Wiley & Sons, Ltd.     

Multiple Transitions in Polyvinyl Alkyl Ethers at Low Temperatures

Abstract

A series of polyvinyl alkyl ethers, with the ether side chain ranging in length from methyl to n-decyl, has been studied from ～60°K to above the glass transition. A linear variable differential transformer was used to measure the linear expansion coefficient, α, complemented by mechanical loss measurements with a freely oscillating torsion pendulum and dielectric loss measurements. In addition to the glass transition, two low-temperature transitions have been observed in these systems. The first below T_g, T_gg(1), follows the same trend as the glass transition, i.e., as the side chain length increases the temperature at which the transition occurs decreases, until n-octyl, where side chain crystallization is manifested. The second transition below T_g, T_gg(2), occurs at ～100°K irrespective of side chain length. Because of their analagous dependence on side chain length, T_gg(1) is thought to be similar to the glass transition, i.e., due to main chain motion. Δα' at T_gg(2) is of greater magnitude than Δα' at T_gg(1) and is thought to be a result of reorientation of the side chain.     

The effects of co-lyophilized polymeric additives on the glass transition temperature and crystallization of amorphous sucrose

The purpose of this study was to measure the effect of co-lyophilized polymers on the crystallization of amorphous sucrose, and to test for a possible relationship between the ability of an additive to raise theT _g of a sucrose-additive mixture, relative to theT _g of pure sucrose, and its ability to inhibit crystallization. Differential scanning calorimetry was used to measure the glass transition temperature,T _g, the non-isothermal crystallization temperature,T _c, and the induction time for crystallization,Q, of sucrose in the presence of co-lyophilized Ficoll or poly(vinylpyrrolidone) (PVP). The effect of these polymers on the crystallization of sucrose was significant as demonstrated by a marked increase inT _c, and in the induction time (Q) in the presence of relatively small amounts (1–10%) of additive. Surprisingly, small amounts of polymeric additive had no effect on theT _g of sucrose, although at higher concentrations, theT _g increased proportionally. Thus, it appears that the inhibition of sucrose crystallization by the additition of small amounts of a higher-T _g component cannot be attributed solely to changes in molecular mobility associated with an increase inT _g.     

Dielectric relaxation of poly(phenylene sulfide) containing a fraction of rigid amorphous phase

The dielectric relaxation behavior of poly(phenylene sulfide), PPS, has been investigated from room temperature to 180°C. This study was undertaken to examine the mobility of the amorphous phase through the glass transition region, to determine the contribution that rigid amorphous phase material makes to the relaxation process. Semicrystalline samples contain a fraction of the rigid amorphous phase, which was determined from the heat capacity increment at the glass transition, using degree of crystallinity determined from x-ray scattering. In the dielectric experiment, we measured the temperature and frequency dependence of the real and imaginary parts of the dielectric function. ε″ vs. ε′ was used to determine the dielectric relaxation intensity, δε = ε_s–ε∞, at temperatures above the glass transition. For amorphous PPS, δε decreases as temperature increases, while for all semicrystalline PPS, δε increases with temperature. The ratio of semicrystalline intensity to amorphous intensity determines the total fraction of dipoles which are already relaxed at a given temperature. Results indicate that more and more rigid amorphous phase material relaxes as the temperature is increased. This provides the first evidence that rigid amorphous phase material in PPS contains chains that possess different levels of molecular mobility. Finally, to the temperature of the loss peak maximum, at a given frequency, we assign the value of the dielectric T_g. For both melt and cold crystallization, the dielectric T_g systematically decreases as the crystallization temperature increases, and as the fraction of rigid amorphous phase decreases.     

Quantitative nanoscale measurements of the thermomechanical properties of poly-ether-ether-ketone (PEEK)

Understanding the internal structure and organization of semicrystalline polymers, especially at the nanoscale, has many challenges for researchers to date. In this article, we demonstrate a quantitative method for investigating the local viscoelastic properties (i.e., storage and loss moduli, as well as loss tangent) of semicrystalline polyether-ether ketone (PEEK) through the combination of contact resonance atomic force microscopy (CR-AFM) and in situ local heating with a thermal probe. Furthermore, the local viscoelastic properties of the crystalline and amorphous phases were decoupled by performing thermal CR-AFM array mapping near the glass transition temperature of PEEK (T_g, 143 °C). A distinct bimodal distribution of tip-sample interaction was observed for PEEK near its T_g, providing a means to estimate the T_g and the degree of crystallinity of PEEK.     

How did we find the rigid amorphous phase?

The first experimental evidence of the existence of the rigid amorphous phase was reported by Menczel and Wunderlich [1]: when trying to clarify the glass transition characteristics of the first main chain liquid crystalline polymers [poly(ethylene terephthalate-co-p-oxybenzoate) with 60 and 80 mol% ethylene terephthalate units] [2], the absence of the hysteresis peak at the lower temperature glass transition became evident when the sample of this copolymer was heated much faster than it had previously been cooled. Since this glass transition involved the ethylene terephthalate-rich segments of the copolymer, we searched for the source of the absence of the hysteresis peak in PET. There, the gradual disappearance of the hysteresis peak with increasing crystallinity was confirmed [1]. At the same time it was noted that the higher crystallinity samples showed a much smaller ΔC _p than could be expected on the basis of the crystallinity calculated from the heat of fusion (provided that the crystallinity concept works). Later it was confirmed that the hysteresis peak is also missing at the glass transition of nematic glasses of polymers. When checking other semicrystalline polymers, the sum of the amorphous content calculated from the ΔC _p at the glass transition, and the crystallinity calculated from the heat of fusion was far from 100% for a number of semicrystalline polymers. For most of these polymers, the sum of the amorphous content and the crystalline fraction was 0.7, meaning that ca. 30% rigid amorphous fraction was present in these samples after a cooling at 0.5 K min⁻¹ rate. Thus, the presence of the rigid amorphous phase was confirmed in five semicrystalline polymers: PET, Nylon 6, PVF, Nylon 66 and polycaprolactone [1]. Somewhat later poly(butylene terephthalate) and bisphenol-A polycarbonate [3] were added to this list.     

Atomistic simulation of the glass transition of di-substituted polysilanes

Molecular dynamics has been used to determine the glass transition temperature of the amorphous phase of five di-substituted polysilanes from plots of specific volume versus temperature. In each case, good agreement was obtained between the simulation values and the reported DSC results. The effect of amorphous cell dimensions and equilibration time on T_g has been investigated. The use of larger cells provides better agreement with experimental T_g and probably more accurate densities as suggested by earlier studies. The effect of pressure on the T_g of two different polysilanes was also investigated. Although experimental data for comparison is unavailable, values obtained for dT_g/dp are consistent with those reported for other polymers. Vectorial autocorrelation analysis was used to explore the mobility of the polysilane main chains and side groups relative to polyalkanes, polyphosphazenes, and polysiloxanes.     

Dielectric relaxations and electrical conductivities of poly(alkyl methacrylates) under high pressure

Dielectric properties of four methacrylate polymers (methyl, ethyl, n-butyl and n-octyl) were studied in the frequency range 0.0001 cps–300 kcps at temperatures above and below the glass transition temperature and at various pressures up to 2500 atm. At temperatures well above T_g a single relaxation peak (α′ peak) was observed in the case of the higher n-alkyl methacrylates. However, this peak was split into two peaks, α and β, with decrease in temperature or increase in pressure. The molecular motions corresponding to the α and the β relaxation processes are the micro-Brownian motions of amorphous main chains and of flexible side chains, respectively. From the temperature and the pressure dependence of the average dielectric relaxation time of these polymers the single relaxation process (the α′ process) was attributed to the micro-Brownian motion of the main chain coupled with that of the side chain. The effects of temperature and pressure on the d.c. conductivity of these polymers were also studied.     

Rigid amorphous fraction in poly(ethylene terephthalate) determined by dilatometry

Volumetric thermal analysis of semicrystalline poly(ethylene terephthalate), PET, with different content of crystalline phase was carried out using mercury-in-glass dilatometry. The effect of crystals on the thermal properties of amorphous phase (glass transition temperature, T _g, thermal expansion coefficients, α) were determined. At cold-crystallization (106°C, up to 4 h), crystalline content of 2.4–25.3 vol.% was achieved. Increasing content of crystalline phase broadens the glass transition region and increases T _g. The change of thermal expansion coefficient during glass transition is lower than that predicted by the two-phase model, which indicates the presence of a third fraction — rigid amorphous fraction (RAF), whose content steadily increases during crystallization. However, its relative portion (specific RAF) is significantly reduced. Further significant decrease in specific RAF appears after annealing at a higher temperature.     

Some aspects for sintering of β-rhombohedral boron

The investigation of sintering conditions for magnesiothermic amorphous boron (Mg_mB_n) powder is presented. The results of chemical and X-ray analyses of magnesiothermic boron (Mg_mB_n) indicate that it consists of amorphous boron MgB₁₂ and a lesser amount of β-rhombohedral boron. The Mg_mB_n-sintering process is determined by the conditions of amorphous boron transformation into β-boron (crystallization), such as the process of decomposition of MgB₁₂ followed by formation of the “new” centers of active elementary boron. As a result of the experimental investigations of this process the following three stages—thermal decompositions, crystallization and Mg_mB_n sintering—were combined into one sintering process with the sintered bodies as a result of it.