A three-phase microstructural model to explain the mechanical relaxations of branched polyethylene: a DSC,WAXD and DMTA combined study

The thermal transitions of well-characterised single-site catalysed polyethylenes having various degrees of short chain branching have been studied by differential scanning calorimetry, X-ray diffraction and dynamic mechanical thermal analysis. A critical discussion based on the results obtained by means of the different techniques is presented. The results suggest that the γ transition is independent of the branching content and degree of crystallinity, pointing towards a sub-glass local relaxation mechanism related to both amorphous and crystalline fractions. The temperature of the β transition, T _β from dynamic mechanical measurements, is in agreement with the glass transition temperature obtained by calorimetry, T _g. Moreover, T _γ, and also T _β are directly related to a change in the thermal expansion coefficient of the amorphous phase observed by X-ray scattering. It is found that the corresponding scattering distance of the amorphous halo depends on crystallinity. In addition, the calorimetric heat capacity values at T _β do not account for the total amorphous fraction determined for each material. The relaxation motions assigned to the amorphous phase glass transition seems to parallel the subsequent melting of the crystalline structure, suggesting a hierarchical motion of different structures as temperature increases. Dynamic mechanical thermal analysis supports these observations, showing a broad transition in the phase angle involving first the relaxation of amorphous phase, then the (presumable) more rigid intermediate phase, and finally the crystalline phase, as the temperature increases.     

Carbon-13 NMR of polyethylenes: Correlation of the crystalline component T1 with structure

The carbon-13 spin-lattice relaxation times T₁ of the crystalline portion of a set of polyethylenes have been studied. Chain structure and crystallization conditions have been varied over the widest possible extremes so that large differences are developed in the level of crystallinity, the supermolecular structure, and the crystallite thickness. Concomitantly, the observed crystalline T₁ values cover the extraordinarily wide range of about 40–4500 s. They bear a one-to-one relation with the crystallite thickness, which is found to be the key structural variable determining this property. A correlation with the temperature for the α-transition can be established, which implies a similar type of segmental motions for the two phenomena. Major changes in the interfacial structure can also have a drastic influence on the value for the crystalline T₁. Analysis of the magnetization decay curve also allows for a quantitative determination of the degree of crystallinity, which is found to be in excellent agreement with the corresponding value found from Raman spectroscopy.     

Thermophysical properties of 1-hexanol over the temperature range from 303 K to 503 K and at pressures from the saturation line to 400 MPa

Isobaric thermal expansivities, _P(T,p), of 1-hexanol have been measured in a pressure-controlled scanning calorimeter from just above the saturation vapour pressure to 400 Mpa at temperatures from 302.6 K to 503.15 K. The specific volume isotherm, v(T_R,p), at T_R=302.6 K has been derived from measurements of isothermal compressibilities up to 400 MPa and from the specific density at atmospheric pressure. Specific volumes, isothermal compressibilities, thermal pressure coefficients, and isobaric and isochoric heat capacities for the whole pressure and temperature range are derived from these data and from literature data on the saturation vapour pressures and on the isobaric heat capacities at atmospheric or saturation vapour pressure.     

WAXD STUDY OF POLYMORPHOUS ISOTACTICPOLYPROPYLENE BY COMPUTER PEAK-RESOLUTION METHOD

Polymorphous i-PP containing a and βphases were studied by WAXD and computer peak-resolution method (CPRM). The asymmetric Gaussian-Cauchy functions (a-GC) were adoptedto fit the profiles of both amorphous and crystalline peaks. For the crystalline peaks, the fitting resultsof a-GC are better than the symmetric-GC; for the amorphous peaks, they are better than polynomialand exponential functions, etc. Using the retarded least-square procedure (RLSP) on microcom-puter the results of peak-resolution are rather satisfatory. For the polymorphous samples containingαand βphases, the relations between phase-state, crystallinity, crystalline size, the ratio of α,βrelativecontent and the crystallization temperature T_c were studied by CPRM. The ratio of α,βrelativecontents obtained by CPRM and Turner-Jones eq. have been carefully compared. There are manyimprovements in this work. A simple estimation method of WAXD peak areas, both for amorphousand crystalline peaki, is suggested.     

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.     

Negative thermal expansion in oriented crystalline polymers

Measurements on the thermal expansivity α_∥ and α_? (along and normal to the draw direction, respectively) have been carried out for a series of oriented polymers with widely different crystallinities (0.36–0.81) and draw ratios (1–20) and over large temperature ranges covering the major amorphous transitions in each case. While α_? increases with temperature, α_∥ tends to decrease sharply above the transition temperature. For highly crystalline polymers, α_∥ decreases to values typical of polymer crystals (?1 × 10^?5 K^?1) and this can be attributed to the constraining effect of the crystalline bridges connecting the crystalline blocks. However, for polymers of lower crystallinity, α_∥ may become an order of magnitude more negative and this remarkable phenomenon is attributed to the rubber–elastic contraction of taut tie-moleucles. Since taut tie-molecules and bridges have drastically different effects on α_∥ at high temperatures, this allows a rough determination of their relative fractions.     

Phase diagram and dielectric relaxation studies of n-octyl-isothiocyanato-biphenyl (8BT) in the crystalline E phase under high pressure

The pressure-temperature phase diagram of n-octyl-isothiocyanato-biphenyl (8BT) in the pressure range up to 250 MPa (2.5 kbar) and the temperature range 250-400 K was established with the aid of DTA. At 1 atm the substance exhibits exclusively CrE polymorphism. At pressures above 190 MPa, the clearing line splits showing an additional phase which is not yet identified. Dielectric relaxation measurements on the CrE phase of 8BT were performed in the pressure range 0.1-120 MPa and the temperature range 304-345 K. A Debye-type relaxation process was observed in the frequency range 100 Hz-1 MHz. The longitudinal relaxation time τ, characterizing the molecular reorientations around the short axis, was analysed with respect to the pressure and temperature, yielding the activation volume, Δ^# V = RT(? ln τ/?p)_T, and activation enthalpy, Δ^# H = R(? ln τ/? T^-1)_p, respectively. The results are compared with analogous data obtained recently for similar compounds having other liquid crystalline phases (N, SmA).     

Crystallinity and crystalline phase orientation of poly(1,4‐cis‐isoprene) from Hevea brasiliensis and Taraxacum kok‐saghyz

Crystallization is studied for poly(isoprene‐1,4‐cis) from Hevea brasiliensis (natural rubber [NR]) and from taraxacum kok‐saghyz, mainly by collecting wide‐angle X‐ray diffraction patterns after processing and stretching. Although rubber samples before stretching are generally fully amorphous, crystallization can be induced in NR samples by processing at room temperature under moderate pressure. This phenomenon is possibly associated with nucleation by saturated fatty acid components. For rubber samples being fully amorphous in the undeformed state, strain‐induced crystallization occurs only at high strain ratios (α > 4), leading to high degrees of crystalline phase orientation (f_c > 0.9 for α = 5). Rubber samples presenting some crystallinity already in the unstretched state, on the contrary, reach much lower degrees of axial orientation, even for high strain ratios (f_c < 0.7 for α = 5). These differences in crystallinity and in crystalline phase orientations produce large differences in stress–strain behavior of the rubber. By room temperature processing, the considered NR samples can also develop an unreported disordered crystalline modification, with low intensity of 120 and 121 reflections. This disordered crystalline modification, which is also maintained after axial stretching procedures, can rationalized by a structural disorder along the b axis, possibly associated with statistical sequences of A⁺TA^? or A^?T A⁺ conformations for poly(isoprene‐1,4‐cis) chains. Copyright © 2016 John Wiley & Sons, Ltd.     

Infrared study of high-pressure crystallization of polyethylene

The high-pressure crystallization of polyethylene in a diamond cell has been studied by infrared spectroscopy. The splitting of the CH₂ rocking band at 720–730 cm^?1 as a function of pressure was analyzed. It was found that pressure alone up to 3 kbar will not change the distance between methylene groups in the unit cell. However, this distance can be shortened by crystallization at this pressure. Intensities of selected crystalline (1176 and 1050 cm^?1) and amorphous (1303, 1352, and 1368 cm^?1) bands were measured on samples before and after high-pressure crystallization, and also on samples of various densities crystallized under atmospheric pressure. The increase in the intensities of crystalline bands and concomitant decrease in amorphous bands, together with density changes, indicate that the crystallinity can be enhanced by crystallization under high pressure. Nevertheless, the crystallinity of polyethylene crystallized at high pressure is comparable with that of polyethylene crystallized at atmospheric pressure at low undercooling for long periods of time.     

A 13C NMR study of the effect of γ-irradiation on the chain dynamics of poly(ethylene oxide)

A comparison of solid-state ¹³C nuclear magnetic resonance (NMR) spectra of virgin and vacuum γ-irradiated poly (ethylene oxide) (PEO) evidences marked differences. The unirradiated PEO shows a well-resolved amorphous resonance and a weak, broad envelope of crystalline resonances, while the irradiated PEO presents well-resolved resonances for both the crystalline and amorphous carbons. Upon recrystallization from the melt both PEO samples yield solid-state ¹³C NMR spectra that are closely similar to that of the virgin, unheated sample. Observation of both melt-recrystallized samples at ?60°C yields similar spectra with well-resolved crystalline resonances. Crosslinking is the predominant chemical change occurring during the γ-irradiation of PEO under vacuum and produces a change in the motional character of the crystalline phase. This change is not the result of a reduction in crystallinity as evidenced by differential scanning calorimetry (DSC) observations. The most probable explanation is that the crosslinks are concentrated at the surface of the crystalline lamellae with a resultant change in the low frequency molecular motions of the crystalline chains. This motional change shifts the T_1p^H such that the crystalline carbon nuclei can now be cross-polarized at room temperature and the resonance linewidth is reduced. Following melting and recrystallization the motional characteristics of the irradiated PEO are nearly identical to those of the unirradiated sample, probably as a result of a redistribution of the crosslinks throughout the amorphous phase during recrystallization.     

Determination of crystallinity in polycaproamide fibers by the method of half-integral intensity

The rectilinear relationship between the integral intensity B_i and fiber density d is used as the basis of a new method for evaluation of the crystallinity index of polyamide fibers (PA-6). To calculate the value of the crystallinity index of the fibers one should have the integral intensity value of the sample and the integral intensities of crystalline and amorphous samples determined under defined instrumental conditions. The method provides a quick and simple means for determining the crystallinity. The method of half-integral intensity may be used only with care when the α-monoclinic polymorphic form dominates in the PA-6 fiber.     

Evolution of crystallinity in photodegraded polyethylene films studied by ft-raman spectroscopy

FT-Raman spectroscopic studies of photodegraded polyethylene films have enabled the evolution of the crystallinity process to be measured. Commercial polyethylene films of M_w=90 000 were exposed in a weathering UV-chamber under known conditions of exposure time and radiant energy. The spectral profiles were modelled using Fourier methods. The relative amounts of the orthorrombic crystalline phase, α_c, the amorphous phase, α_a and the interphase, α_b, were calculated using Raman bands at 1416 cm⁻¹ characteristic of the crystalline phase and the bands at 1080, 1305 cm⁻¹, characteristic of the amorphous phase. The interphase content can be calculated from the relationship αb= 1-(α_c+α_a). It was found that the weathering process affects only the relative intensities of the bands attributed to crystalline and amorphous fractions; the crystalline content increases at the expenses of the amorphous fraction. These results are discussed in terms of the changes in the intermolecular forces caused by radiation exposure.     

Solid-state coextrusion of poly(ethylene terephthalate). II. Drawing of semicrystalline PET

The drawing of semicrystalline (33 and 50%) poly(ethylene terephthalate) (PET) films has been studied by solid-state coextrusion. Because of its brittleness and opacity, isotropic and semicrystalline PET film is of little practical use. Early attempts to cold-draw crystalline films led to fracture in contrast to deformation of amorphous PET. However, we have succeeded in systematically preparing films with extrusion draw ratios ≤4.4 from semicrystalline PET. In many cases, the properties of the drawn extrudates, as a function of extrusion temperature T_ext and extrusion draw ratio EDR, were similar to those prepared from amorphous PET. However, some remarkable differences have also been found. In the case of coextrudates prepared from isotropic 50% crystalline PET, we found that the larger the deformation, the lower the apparent resulting crystallinity. In the extreme, a 34% reduction in crystallinity after deformation was observed. For the coextrudates drawn from initially 33% crystalline PET, slightly different behavior occurred. For T_ext ≤ 90°C, all extrudates showed crystallinities lower than the original isotropic film, with a minimum at EDR = 3; for T_ext ≥ 110°C, crystallinities were slightly greater than in the original film and increased with EDR. Qualitative measurements of heats of fusion were in agreement with density gradient results for PET crystallinity. In contrast is our previous finding that extrudates from initially amorphous PET always increase in crystallinity with EDR, because of stress-induced crystallization. The results now suggest that in the T_ext range investigated, the initial spherulitic structure is at least in part destroyed on drawing. In addition, the percent crystallinity is revealed to be dependent on T_ext, with lower values at lower temperatures. Mechanical tests show that the extrudates are similar or sometimes higher in tensile modulus when compared to amorphous PET drawn under the same conditions.     

Microhardness studies of chain-extended PE. I. Correlations to microstructure

The hardness–microstructure correlation of various polyethylene (PE) samples crystallized at high pressure from the melt (chain-extended), with different molecular weights, has been investigated and compared to melt crystallized samples at atmospheric pressure (chain-folded). The hardness, H, of melt crystallized PE is confirmed to increase linearly with the logarithm of the annealing time, t_a, at a constant annealing temperature. The H increase with t_a is discussed in terms of the crystallinity and crystalline lamellar thickness variation. Unusually high hardness values are obtained for samples crystallized or annealed at high pressure as a consequence of the resulting high degree of crystallinity and large crystalline lamellar thickness values. However, it is shown that the high surface free energy value of the chain-extended crystals considerably lowers the hardness values from that of an ideal infinitely thick PE crystal. Analysis of the crystal hardness and the melting temperature data of different polymeric materials emphasizes the close existing relationship between both quantities. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3151–3158, 1999     

Degradation of polymers and morphology: Photo-oxidative degradation of isotactic polystyrene in presence of sulfur dioxide as function of polymer crystallinity

The photo-oxidative chain scission of isotactic polystyrene films has been studied as a function of the degree of crystallinity, SO₂, and NO₂ pressures, and temperature. The rate of chain scission increases in the presence of SO₂ with extent of crystallinity. It is assumed to be faster due to strain in and near the folds in the crystalline areas than in the amorphous regions. In the presence of NO₂, chain scission increases up to about 8% crystallinity but subsequently becomes constant with further increase in crystallinity. It is suggested that the diffusion rates of oxygen and nitrogen dioxide into the films decrease with increasing crystallinity. These two processes compensate each other.     

A dielectric study of molecular relaxation in oxidized and chlorinated polyethylenes

A study was made of the dielectric relaxation in polyethylenes rendered dielectrically active through oxidation (0.5–1.7 carbonyls/1000 CH₂) and chlorination (14–22 Cl/1000 CH₂). Both linear and branched polymers were studied. All of the relaxations between the melt and ?196° were studied in the frequency range 10 Hz to 10kHz (100 kHz in the chlorinated samples). In the linear samples a wide range of crystallinities was studied (55% in quenched specimens to 95% in extended-chain specimens obtained by crystallization at 5 kbar). As is consistent with its being a crystalline process, the α peak was found to discontinously disappear on melting of the samples and reappear on recrystallizing on cooling. The disappearance of the smaller crystals before the larger ones appeared to be evident in the isothermal loss versus frequency curves. The relaxation strength of the α process increases with crystallinity. The measured relaxation strength is less than that expected on the basis of direct proportionality to the crystalline fraction with full contribution of all dipoles in the crystalline material. However, the intensity is not sufficiently low for the process to be interpreted in terms of reorientation of localized conformational defects in the crystal. The variation of intensity with crystallinity is best interpreted in terms of full participation of crystalline dipoles but with selective partitioning of both carbonyls and chlorines favoring the amorphous domains. A strong correlation of the α loss peak location (T_max at constant frequency or log f_max at constant T) with crystallinity for both carbonyl and chlorine containing polymers was found. This variation is interpreted in terms of chain rotations in the crystal where the activation free energy depends on crystal thickness. The dependence of log f_max and T_max on lamellar thickness as well as a comparison with the loss peaks of ketones dissolved in parafins indicates that the chain rotation is not rigid and is accompanied by twisting as the rotation propagates through the crystal. In agreement with previous studies the β process is found to be strong only in the branched polymers but can be detected in the chlorinated linear polymer. The β process was resolved from the α in the branched samples by curve fitting and its activation parameters determined. The γ relaxation peak in oxidized polymers including its high asymmetry (low-temperature tail) and increasing ε_max with increasing frequency and temperature when plotted isochronally can be interpreted in terms of a simple nearly symmetrical relaxation time spectrum that narrows with increasing temperature. No increase in relaxation strength with temperature was found. The chlorinated polymers behave similarly but appear to have some Boltzmann enhancement (450–750 cal/mole) of relaxation strength with temperature. The dependence of relaxation strength on crystallinity indicates that the process is an amorphous one. Further, no evidence of relaxation peak shape changes with crystallinity that could be interpreted in terms of a crystalline component in addition to the amorphous one was found. The comparison of the γ relaxation strength with that expected on the basis of full participation of amorphous dipoles indicates that only a small fraction (～10% in oxidized linear polymers) of them are involved in the relaxation. Thus it would seem that a glass–rubber transition interpretation is not indicated but rather a localized chain motion. It is suggested that the γ process, including its intensity, width, and activation parameters, can be interpreted in terms of an (unspecified) localized conformational (bond rotation) motion that is perturbed by differing local packing environments. The thermal expansion lessens the effects of variations in packing and leads to narrowing with increasing temperature. The conformational motion itself leads to increase in thermal expansion and hence a transition in the latter property. Some previously proposed localized amorphous phase conformational motions appear to be suitable candidates for the bond rotation motion. A weak relaxation peak found at temperatures below the γ and at 10 kHz may possibly be the dielectric analog of the δ cryogenic peak found previously mechanically at lower frequencies.     

Component analysis of proton NMR spectra of linear polyethylene in the α-transition temperature range and phase distribution

Broad-line ¹H NMR spectra of linear polyethylene at temperatures in the α-transition range can be analyzed in terms of contributions from the crystalline and noncrystalline components provided molecular motion in the crystalline region is adequately considered. The spectrum of solid n-C₃₂H₆₆ or n-C₄₄H₉₀ prior to melting is used to take account of the contribution of the crystalline region of the polymer to molecular motions. The temperature dependence of the component distribution in the polymer is briefly discussed for a wide range of temperatures, together with previously reported results at low temperatures. The noncrystalline component is in a rigid glassy state at very low temperatures but with rising temperature it transforms to a mobile glassy state with restricted molecular motion, and transforms partially to the rubbery state at high temperature. The crystalline component remains rigid at low temperature, but some molecular motion is associated with it at higher temperatures in the α-transition range.     

The effect of crystallinity and crosslinking on the depression of the glass transition temperature in nylon 6 by water

The influence of crystallinity and crosslinking on the depression of the glass transition temperature in nylon 6 by water has been investigated by dynamic mechanical methods. Radiation crosslinking by high-energy electrons was effective in preventing morphological changes during the measurement of the incremental change in heat capacity (ΔC_p) at T_g, which was performed by differential scanning calorimetry. The experimentally determined value of ΔC_p, when normalized to account for the crystalline phase, was found to deviate from a linear two-phase relation and was reduced further than would be expected based on this model. It is proposed that nylon 6 is best described by a three-phase model which consists of a crystalline domain, a wholly amorphous domain, and an “intercrystalline” region. The importance of this in explaining the relatively large depression of T_g by small quantities of water is illustrated by applying equations derived to account for the compositional dependence of T_g in polymerdiluent mixtures, based on a classical thermodynamic interpretation of the glass transition phenomenon.     

A pulsed field gradient NMR study of diffusion in semicrystalline polymers: self-diffusion of alkanes in polyethylenes

By means of the pulsed field gradient NMR technique the self-diffusion of six alkanes (from n-butane to n-pentadecane) in three low density polyethylenes and one high density polyethylene differently thermally treated was examined. The concentration dependence could be described very satisfactorily with the free volume theory in the form of Fujita (Adv. Polymer Sci. 3(1961) 1). The parameter B of the diffusants and the fractional free volumef ₂ of the polyethylenes were determined from the experimental data. The fractional free volumesf ₂ show a strong dependence on the type of polyethylene, the main influence results from the different content of CH₃ groups or short chain branches. The diffusion coefficient extrapolated to zero diffusant concentration is proportional to the eighth power of the amorphous content. This strong dependence shows that the free volumes of the amorphous parts of the polyethylenes are intimately connected with crystallinity, both determined by the different degrees of short chain branching. The pre-exponential factor in the free volume expression decreases with increasing amorphous content of the polyethylenes and increases with increasing length of the diffusants. It was found that the spherulite boundaries in the polyethylenes do not act as diffusion barriers.