The advent of computer‐supported IR spectrometers, especially Fourier‐transform IR interferometers, made possible considerable progress in the fine‐structure elucidation of the seemingly well‐known polyethylene and ethene copolymers. This paper gives a review on the results obtained in several laboratories in the following fields: identification of side‐groups and olefinic structures, characterization of ethene homo‐ and copolymers made with different catalysts, by spectral enhancement, difference and derivative spectroscopy, and measurements of the dichroism of oriented samples. In one chapter, recent results with amorphous poly(alkyl ethylene)s obtained with a modified Phillips catalyst are reported.

By means of stepwise crystallization or heat treatment of high-pressure polyethylene samples with several melting peaks can be produced. The melting curves truly reflect the thermal pre-history of the sample, and during melting a distinct "thermal memory effect" can be observed. The stepwise heat treatment produces individually and independently melting crystallites. The thermal and mechanical stability of crystallites formed by stepwise heat treatment has been studied. The crystallites are stable below the temperature of heat treatment, and also possess a remarkable mechanical stability, retaining the original thermal properties even under very high specific elongation. The change in the character of melting curves as a function of elongation is in unanimous correlation with the structural changes taking place under stretching. The memory effect, strongly expressed in the case of high-pressure polyethylene, can be traced back to the irregular, branched chain structure.The structures and properties of polymers are greatly influenced by the conditions of crystallization and the thermal and mechanical pre-history of the polymers [1]. Their thermal properties and melting characteristics are also affected by the above factors.Polymer samples prepared under certain conditions have frequently been observed to give melting curves containing several peaks [2][3][4][5][6][7]. Thus, the stepwise heat treatment of polyethylene crystallized from melts [2], or polypropylene [3], or rolled and stretched polyethylene foils [4,5] may produce samples the melting curves of which have several peaks. However, different opinions exist as regards the origin of the melting peaks of heat-treated high-pressure polyethylene [2,4,5].As reported in our previous papers [6,7], an arbitrary number of melting peaks can be caused to appear in the melting curve of high-pressure polyethylene by applying the method of stepwise isothermal crystallization. We have found that the peaks of the melting curves are due to melting and recrystallization processes [7], and that the melting of high-pressure polyethylene faithfully reflects the thermal history of the sample. This phenomenon was given the name "thermal memory effect". This paper deals with the characteristic features of the melting of high-pressure polyethylene subjected to stepwise heat treatment, and with the interpretation of the peaks of the melting curves.

Stepwise crystallization and successive melting of Tipolen low density polyethylene was studied by DSC. As a result of the stepwise crystallization of the polyethylene, there were as many peaks on the melting curve as the number of steps used for the crystallization of the sample. Similar results were obtained on Tipolen lowdensity polyethylene fractionated by gel permeation chromatography, and on Celene low-density polyethylene, too.Crystallization and melting processes of polymers show many features different from those of materials of low molecular weight [1,2]. It is well known that the crystallinity of polymers depends on the temperature, and their melting processes take place in wide temperature intervals. Experimentally determined melting processes and melting point of polymers depend on conditions of previous crystallization. These features are attributed to the macro-and polymolecular nature of they polymers, to the significant role of kinetic factors in their crystallization process, as well as to morphological characteristics of the polymer crystals [3].It is assumed that a great number of crystallites with different melting points are formed during the crystallization process. Richardson et al. [4] studied in detail melting characteristics of polymethylene copolymers containing short branches. The samples in their investigation were made by stepwise crystallization. In order to be able to reach thermodynamic equilibrium conditions, the samples were held at each temperature for a long time (some days). Mentioned authors have established that the melting point is decreased and the melting interval is increased as a consequence of introducing comonomer units. This paper deals with the influence of the crystallization conditions and thermal prehistory of polymers on their melting. It is experimentally proved that the separation of crystals with different melting points is possible in their melting processes. ExperimentalIn most of our experiments non-fractionated Tipolen FA 2210 low density polyethylene (produced by Chemical Works of Leninvfiros, Hungary) was used (m. p. 388 K; CHa/1000 C = 15.3; M w = 148 000). Some experiments were made using fractions of Tipolen PE produced by gel permeation chromatography

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The melting conditions of biaxially oriented polypropylene (BOPP) have a very strong effect on the courses of both non-isothermal and isothermal crystallization from the melt. It has been shown that the crystallization proceeds by two mechanisms. Depending on the melting conditions, both crystallization mechanisms can proceed individually or simultaneously (double crystallization). The high rate mechanism, occurring in non-isothermal crystallization at low undercooling and observed in isothermal crystallization, can be attributed to the ordered structure of the melt preserved after melting, presumably a mesomorphic state. The second component corresponds to crystallization from the isotropic melt.Anomalous crystallization of BOPP samples can be eliminated by energetic heat treatment, by the destruction of ordered structures. As a result of this treatment, the characteristics of crystallization and its quantitative parameters will become similar to those of unoriented polypropylene.The characteristics of the crystallization of isotactic polypropylene (PP) have been studied by several research groups [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. It has been established on the basis of the dilatometric [1 -4, 6-8], optical polarization [5,10] and calorimetric [9, 10] investigations of isothermal crystallization that the crystallization isotherms can be described by the Avrami equation. Moreover, it has been observed that the rate of crystallization and its quantitative characteristics depend strongly on the thermal history of the samples [3,4,7,15].Our present work reports on the effect of the thermal history (melting conditions) of biaxially oriented polypropylene samples (BOPP) on the courses of isothermal and non-isothermal crystallization from the melt. A new type of crystallization memory effect, the phenomenon of "double crystallization", has been detected in these investigations. ExperimentalSamples cut out from biaxially oriented polypropylene flasks manufactured by two-step technology (Hercules Inc.) have been used for the investigations. The basic substances is PP of Pro-Fax 6723 type. According to X-ray analysis made with a Mfiller Micro 111 instrument, the a-modification of PP is present in the samples, and the orientation along the two axes is homogeneous. On the basis of our thermorelaxation investigations, the elongation during orientation is 300 -400 %.