The structure of copolymer crystals formed from dilute solution and in bulk

The thermodynamic and structural properties of a series of hydrogenated polybutadienes, crystalized in the bulk and from dilute solution, have been investigated. These polymers are ethyl-branched ethylene copolymers with narrow molecular weight and composition distributions. Despite the fact that for both modes of crystallization these random-sequence copolymers display a lamellar crystalline habit, all of the independent physical chemical measurements are quantitatively consistent in indicating a relatively thin crystallite with a large amorphous disordered overlayer. This is seen to be a very general phenomenon for copolymer crystallization. The core thicknesses, determined from Raman LAM and from small-angle x-ray scattering, are in good agreement. Quantitatively consistent values of the degree of crystallinity are obtained from the density, enthalpy of fusion, Raman internal modes, Raman LAM, and small-angle x-ray scattering. It is significant that independent thermodynamic and structural methods give the same result.     

The heat of fusion of poly(4-methyl pentene-1)

We have combined pressure–volume–temperature measurements and high-temperature x-ray diffraction to calculate the heat of fusion of the poly(4-methyl pentene-1) crystal from the Clapeyron equation. We find ΔH_f = 14.8 cal/g, only about half as large as several literature values. The new value of the heat of fusion leads to increased degrees of crystallinity, in agreement with the highly crystalline appearance of the x-ray patterns of typical samples, in particular oriented fibers. The new entropy of fusion at constant pressure of 2.46 cal/deg per mole of monomer units is significantly lower than the values for polypropylene, polybutene-1, and polypentene-1, indicating possible ordering in the melt of this polymer.     

Evaluation of the crystallinity of copolyamide fibers

Density and x-ray diffraction measurements of the degree of crystallinity have been made on copolymers of polyamide 6 PA-6 with hexamethylenediamine terephthalate and ethylenediamine terephthalate ranging composition from 4 to 35 mole-% of the terephthalates. The degree of crystallinity can be determined from the ratio of the sum of mass contribution products of the crystalline regions for each of the comonomers to the total copolymer mass. Such a procedure can be used when the molar fractions of comonomers in copolymer are known. The results obtained are in agreement with the theoretical values.     

工业用涤纶纤维的熔融行为与某些结构参数关系的研究

用ＤＳＣ法研究了国内外不同厂家生产的工业用ＰＥＴ纤维的熔融行为,其热谱表明,一般呈现熔融双峰．实验证明,双熔融峰是由ＤＳＣ过程中纤维材料中一部分热稳定性差的结构发生重排所致．结合用ＷＡＸＤ法、ＳＡＸＳ法、双折射法、密度法获得的结构参数指出,工业用纤维的熔融行为,除受晶粒尺寸、晶粒完善程度、晶粒尺寸分布等因素的影响之外,连续网的结构也起着重要作用．另外,用ＤＳＣ法测定工业用纤维结晶度的精度还有不如人意之处．     

Structure and property relationships in ethylene–vinyl acetate copolymers

The effects of vinyl acetate content on crystallinity of ethylene–vinyl acetate (E/VA) copolymers were investigated by x-ray diffraction and differential thermal analysis (DTA). The values of these parameters obtained from DTA were found to agree quantitatively with data calculated from x-ray, probability equations, and copolymer theory. The melting points of the crystalline copolymers, and the molar amounts of vinyl acetate to produce a completely amorphous rubber corresponds exactly to that predicted by the Flory theory. The random character expected in E/VA copolymers is thereby confirmed. The physical properties of E/VA copolymers of all ranges of compositions and crystallinity were determined. Depending directly upon vinyl acetate content, the copolymers changed progressively from highly crystalline polyethylene to semicrystalline polyethylene, a completely amorphous rubber, a soft plastic with a glass transition near room temperature. Properties which were correlated with copolymer composition include: crystallinity, melting point, density, modulus, tensile strength, glass transition, and solubility. Finally, the effect on crystallinity and physical properties of replacing the acetoxy group in E/VA with the smaller, highly polar hydroxyl group (ethylene—vinyl alcohol copolymer) was also investigated.     

Gas transport properties of thermotropic liquid-crystalline copolyesters. I. The effects of orientation and annealing

Gas transport properties are reported for two series of films prepared from copolyesters of 73 mol % hydroxybenzoic acid (HBA) and 27 mol % 2,6-hydroxynaphthoic acid (HNA) which systematically vary the degree of orientation and annealing time. Scanning electron microscopic (SEM) photomicrographs of the liquid-crystalline polymer (LCP) films showed evidence of a skin-core structure and polydomain texture. The degree of orientation in the films was quantified by analyzing the azimuthal intensity of the x-ray reflection associated with the lateral packing of the nematic mesophase. Using heat of fusion data from differential scanning calorimetry (DSC), the films were found to contain low levels of crystallinity estimated to be in the range of 5 to 15 wt %, which increased with annealing time. Permeability measurements were made for He, H₂, O₂, N₂, Ar, and CO₂ at 35°C and the diffusivities were computed from time-lag data. The films exhibited excellent barrier properties resulting largely from very low gas solubility coefficients. A moderate reduction in permeability was observed with increased orientation, which could be attributed directly to a decrease in the effective diffusivity. The effect of increased crystallinity from annealing on the permeability coefficients was smaller than would be expected for similar changes in the crystallinity of conventional polymers. Analysis using a simple two-phase model suggests that a mechanism dominated by transport in a small volume fraction of boundary regions possibly could account for the bulk transport properties of these materials.     

State of order in isotactic polyhexylethylene

Cooling slowly from the melt and annealing at 250 K for a month isotactic polyhexylethylene becomes partially crystalline. The degree of crystallinity, calculated according to the method of Hermans and Weidinger, is 0.3. The wide-angle x-ray diffraction diagram shows seven reflections, not completely independent of one another. These x-ray data do not allow an exact determination of the structure. The main chins possibly form quaternary helices (e. g., 4/1 or 7/2 helices) and the side chains are arranged in triclinic structure. The unit cell may be monoclinic: a ～ 0.56 nm, b ～ 3.8 nm, c ～ 0.76 nm (in case of a 4/1 helix), β ? 97° The melting temperature is 278 K and the heat of fusion amounts to 30 J/g.     

Influence of uniaxial draw on near-surface structure in poly(ethyleneterephthalate)

Relative amounts of surface crystallinity and amorphous structure and their respective surface orientations were examined as a function of depth from the surface for a series of uniaxially drawn poly(ethyleneterephthalate) (PET) films using polarized attenuated total-reflectance infrared spectroscopy (ATR-IR). Qualitative depth resolution on a scale from ca. 0.5 to 5 μm is obtained. The results indicate a higher degree of extended trans segments and a higher degree of extended trans orientation near the surface in the machine direction, relative to the bulk film. These quantities also increase with increasing uniaxial draw ratio. No evidence for orientation of the gauche conformer at the surface as a function of uniaxial draw ratio is observed. All conclusions are consistent with the presence of increased crystallinity at the surface of the films. Increases in trans structure and orientation measured by ATR-IR measurements are consistent with bulk crystallinity measurements from density and x-ray scattering measurements.     

Characterization of selectively degraded poly(ethylene terephthalate)

To investigate the morphology of unoriented poly(ethylene terephthalate) (PET) films and the selective character of the aminolysis of PET, 67% crystalline polymer samples were degraded with 40% aqueous methylamine at room temperature. The aminolyzed PET samples were subjected to gel permeation chromatography (GPC), viscometry, electron microscopy, and small-angle x-ray diffraction (SAXD). Weight loss and density crystallinity measurements were also made. After 24 hr of aminolysis, the amorphous regions and chain folds were completely removed. The long molecular chains in the semi-crystalline polymer were reduced to monodisperse rods having a molecular weight of 1,800. The corresponding lamellar thickness was calculated to be 101 Å, consistent with the x-ray diffraction and electron microscope (EM) measurements. The EM photographs of “stripped” crystals show the lamellar structure previously found for other selectively degraded polymeric materials. The weight of crystalline debris remaining was consistent with the initial crystallinity. After degradation the crystallinity as determined by density was 96%.     

Tetrachlorobisphenol-A adipate polymers. I. Crystallization of poly(tetrachlorobisphenol-A adipate)

The properties of melt-crystallized poly(tetrachlorobisphenol-A adipate) were studied by using a differential scanning calorimeter. The dependence of melting point and the degree of crystallinity are reported as a function of the crystallization conditions. The heat of fusion is equal to 8.1 kcal/mole, while the equilibrium melting point, as determined by extrapolation, is 283°C. The polymer crystallized from the melt has a maximum degree of crystallinity of 0.53.     

X-ray determination of crystallinity and orientation in poly(ethylene terephthalate)

The measurement of crystallinity and orientation in poly(ethylene terephthalate) is discussed. A simple procedure is given for the estimation of orientation from the (1 05) plane, and it is shown that methods which use the equatorial planes are subject to certain disadvantages. In addition a method is given for the measurement of x-ray crystallinity. The technique is applied to fibers and films of various treatments and a linear relation is found between density and x-ray crystallinity.     

聚全氟乙丙烯树脂中六氟丙烯分布问题的探讨

<正> 聚全氟乙丙烯树脂(以下简称FEP)是四氟乙烯与六氟丙烯的共聚物.目前对FEP尚无适当溶剂,所以不能用C~(3)高分辨NMR测定共聚单体在共聚物中的分布,故采用X射线衍射方法测量晶格常数和结晶度,以及密度与熔点的测定,对国产不同聚合体系FEP中的六氟丙烯分布问题进行探讨. 图1比较了聚四氟乙烯(PTFE)及FEP的X射线衍射谱(23℃)。在FEP共聚物中除了(100)晶面衍射峰外,尚有微弱的(110),(200)等晶面衍射峰,可以看出,FEP的行     

Polyolefins - poly(ethylene oxide) blends: Structure and properties

Structure and properties of polyolefins-poly(ethylene oxide) (PEO) blends were studied by combination of thermal analysis, light microscopy and fluorescence probe technique. Mixing and preparation conditions influence the calorimetric behavior, changing heat and temperature of melting and crystallinity degree of blended polymers. Values of the static mechanical parameters as well as flow characteristic parameters of the blends are lower for the higher PEO fraction in the samples. Fluorescence investigation of pyrene (changes in the vibrational fine structure) showed a nonhomogeneous distribution of the probe molecules between polyolefin and PEO environment. From these data the partition coefficient between polyolefin and PEO phase can be calculated and correlated with the crystallinity changes of the mixture composition.     

The enthalpy of fusion and degree of crystallinity of polymers as measured by DSC

Crystalline polymers are not in thermal equilibrium and thermodynamic parameters such as enthalpy of fusion as determined by differential scanning calorimetry from the area under the melting endotherm over a wide temperature range have not been measured under equilibrium conditions. Accordingly measurements of the degree of crystallinity based on the enthalpy of fusion reflect experimental conditions, are incorrect in that they do not usually agree with those determined by other analytical procedures, such as density and WAX scattering, particularly when measured at ambient temperatures. While this has been repeatedly pointed out procedures used to determine the fractional crystallinity of polymers based on the enthalpy of fusion continue to be widely used.Using the First Law method, the enthalpy of fusion and fractional crystallinity have been measured as a function of temperature for metallocene polyethylene and polyethylene terephthalate during heating. The crystallinity measured on the sample prior to heating is in good agreement with that determined by density and wide angle X-ray diffraction.     

Structural studies of radiation-crosslinked poly(ethylene oxide)

The structure and thermal behavior of freeze-dried gels of radiation-crosslinked high molecular weight poly(ethylene oxide) (PEO) were investigated by optical and electron microscopy, wide-angle x-ray scattering (WAXS), DTA, TGA, and thermomechanical analysis. The gels are highly porous with thin crystalline walls. Small spherulite and hedrite structures are observed on the walls. A model for gel formation in solution is suggested. A statistically homogenous chemical network is formed as a result of intrachain and interchain crosslinking. Simultaneous grafting of macromolecular fragments formed by chain scission also occurs. On increasing the irradiation dose from 1 to 15 Mrad, the degree of crystallinity determined by x-ray diffraction and the total intensity of diffraction gradually decrease. The temperature and enthalpy of melting diminish steeply up to 5 Mrad, fall only slightly from 5 to 8 Mrad, and do not change from 8 to 15 Mrad. By comparing the x-ray and DTA crystallinity values, this is shown to be due not only to reduced crystallinity at higher network density but also to Tree energy changes of entropic origin in crystalline and amorphous regions. Radiation chemical yields, G(-units), for these dose ranges are 100, 38, and 0, respectively. Thermomechanical analysis was used to determine the elastic modulus of compression as a function of the dose absorbed, and the average molecular weight of network chains was estimated. decreases with doses up to 10 Mrad and does not change with further irradiation.     

Microstructural characterization of semicrystalline copolymers by Raman spectroscopy

We employ Raman spectroscopy to characterize several microstructural aspects of a family of ethylene-propylene copolymers (EPC). Focus is made on the simultaneous analysis of crystallinity and chemical composition. A curve fitting procedure is used to isolate Raman bands ascribed to polypropylene chains in the crystal lattice from contributions of the amorphous phase. Crystal contents of EPC calculated on this basis are in the range 10–34 wt%, in good agreement with independent wide angle x-ray diffraction and differential scanning calorimetry measurements. Besides, Raman spectroscopy captures in some of the samples a mixed crystalline structure with both, polyethylene and polypropylene crystals, indicating a distinctive molecular architecture. The chemical composition of EPC is obtained from Raman spectra in the melt state to decouple peaks characteristics of the crystal lattice from fundamental vibrational modes of the polymer chain. EPC present ethylene contents in the range 5–26 mol%, in good agreement with parallel results from ¹³C nuclear magnetic resonance analysis. Remarkably, a rather complete characterization of EPC can be achieved on the base of a single experimental technique.     

The heat of fusion of polytetrafluoroethylene

The heat of fusion of virgin and melt-processed polytetrafluoroethylene (PTFE) was determined using the Clapeyron equation. Experimental data were obtained from PVT experiments and high-temperature x-ray diffraction measurements. For virgin, as-polymerized PTFE, the melting temperature is given by where, for T_m in degrees Celsius, A = 346.3±1.2, B = 0.095±0.003, and P is the pressure in kilograms per square centimeter. At the end of the atmospheric-pressure melting interval, the amorphous and crystalline specific volumes V₁ and V_c are 0.6517 and 0.492 cm³/g, respectively. Thus the heat of fusion is 24.4 cal/g, or nearly twice the value reported previously. The increases in enthalpy and volume at the melting point both indicate a degree of crystallinity of about 75–80% although infrared, x-ray, and NMR data give much higher levels. Data from calorimetry, NMR, and dynamic mechanical measurements indicate that in virgin PTFE some of the crystals continue to experience torsional oscillations at temperatures below the room-temperature transitions. This indicates that there are at least two kinds of crystalline regions. For previously melted PTFE, T_m is determined by A = 328.5±0.7 and B = 0.095±0.002, the volumes are V_am = 0.6349 and V_cr = 0.4855 cm³/g, and the heat of fusion is 22.2 cal/g. The entropy of fusion for PTFE is much closer to that of polyethylene than was previously believed.     

Rheo-optical studies of high polymers. XVIII. Significance of the vertical shift in the time-temperature superposition of rheo-optical and viscoelastic properties

To determine the true reason for the increase in birefringence and the decrease in relaxation modulus for high-density polyethylene with rising temperature, changes in crystalline structure as well as in thermal, viscoelastic, and rheo-optical properties with temperature were measured, by several techniques, including DSC, DLI, infrared dichroism, x-ray diffraction, and NMR. The values for degree of crystallinity obtained from the DSC fusion curve, density, and infrared absorbances coincide very well and show almost no divergence till about 80°C. The optical vertical shift factor pT can be related to the ratio of the orientation function for the crystal c axis at an arbitrary temperature to that at the references temperature, f_ε/f_ε0. The mechanical vertical shift factor b_T, on the other hand, is associated with the temperature dependence of the mobile fraction, as determined by NMR measurements, but not with variations in degree of crystallinity.     

Ethylene/1-Hexene Copolymerization and Synthesis of LLDPE/Nanocarbon Composite through In Situ Polymerization

Copolymerization of ethylene/1-hexene using a modified ZN-type catalyst was carried out in the presence of triethylaluminium as cocatalyst. The optimum copolymerization activity was obtained at Al: Ti = 357: 1, 60°C and the comonomer concentration of 0.6 mol/L in the range studied. Copolymer/nanocarbon (including multiwalled carbon nanotube, graphene nanoplatelet) composites were prepared via in-situ polymerization. The copolymerization activity decreased by addition of the nanocarbon into the reactor. The presence of graphene nanoplatelet in nanocomposites reduced the melting temperature and increased heat of fusion, crystallinity and density of the obtained polymer. In the copolymer/carbon nanotube nanocomposites, decreasing of melting temperature was observed in comparison to pure copolymer, whereas, heat of fusion, crystallinity and density increased. The results of TGA analysis showed that the addition of nanocarbons has improved the thermal stability of obtained copolymers.     

The degree of crystallinity of monoclinic isotactic poly(propylene)

The degree of crystallinity of a set of monoclinic (alpha) isotactic poly(propylenes), prepared by a metallocene‐type catalyst, were determined at room temperature. Three different methods were used: density, enthalpy of fusion, and wide‐angle X‐ray scattering, and the results compared. The relation between the heat of fusion and the specific volume of these poly(propylenes) was found to be nonlinear, thus precluding any linear extrapolation to obtain the heat of fusion of the pure crystal (ΔH_u). The value of ΔH_u obtained from depression of the melting temperature by diluents is used. Based on the unit cell density of monoclinic crystals formed from a low defected fraction, the density obtained crystallinity levels were found to be between 0.l5–0.25 higher than those calculated from the heat of fusion. This relatively large difference holds for the isothermally crystallized and quenched isotactic poly(propylenes), and reflects the contribution of the interphase to the density determined crystallinity, which does not contribute to the heat of fusion. Paralleling results found in other systems, the crystallinity levels obtained from wide‐angle X‐ray scattering agree with those obtained from density, indicating a significant contribution of the partially ordered phase to the total diffraction. Emphasis is given on the need to account for the large differences in the crystallinities of poly(propylene) measured by different techniques when evaluating the dependence of properties on this quantity. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 323–334, 1999