Effect of methyl groups on the thermal properties of polyesters from methyl substituted 1,4-butanediols and 4,4'-biphenyldicarboxylic acid

Results are reported on the effect of lateral methyl groups on the thermal properties of a series of polyesters prepared from diethyl 4,4-biphenyldicarboxylate and various methyl substituted 1,4-butanediols. The diols were 1,4-butanediol; 2-methyl-1,4-butanediol; 2,2-dimethyl-1,4-butanediol; 2,3-dimethyl-1,4-butanediol; 2,2,3-trimethyl-1,4-butanediol; and 2,2,3,3-tetramethyl-1,4-butanediol. Apart from the tetramethyl derivatve, the transition temperatures of the methyl substituted polyesters were lower with respect of the unsubstituted polyester. On the basis of polarized photomicrographs, a smectic A mesophase was found for the unsubstituted polyester, whereas a nematic mesophase was observed for the 2-methyl substituted polyster. The 2,2-dimethyl, 2,3-dimethyl, and the 2,2,3-trimethyl substituted polyesters showed no liquid crystalline behavior. The 2,2,3,3-tetramethyl derivative displayed a birefringent melt phase although the DSC measurements were not unambiguous. A copolyester based on diethyl 4,4-biphenyldicarboxylate, 1,4-butanediol, and 2,2,3,3-tetramethyl-1,4-butanediol showed a broad nematic mesophase. Further evidence for the nematic mesophase of this copolyester and the 2-methyl substituted polyester was provided by dynamic rheological experiments. Based on thermogravimetric analysis, it was concluded that the thermal stability was affected only when four methyl side groups were present in the spacer

Pivalolactone, 1 interchange reactions with polypivalolactone

Ester interchange, alcoholysis, and acidolysis of polypivalolactone (PPVL) were studied by melting PPVL with bisphenol diacetates, 1, 4-butanediol, or aromatic diacids. Interchange of PPVL with the diacetates and the diol occured readily, in particular in the presence of a titanium catalyst. Melting PPVL with 10 mol-% of bisphenol-Adiacetate in the presence of 0,5 wt.% tetrabutylorthotitanate resulted in an incorporation of 33% of the diacetate in the polymer chains, whereas the logarithmic viscosity number decreased by 81%. The ester interchange was suggested to proceed by an initial cleavage of ester bonds in the polymer chain of PPVL, resulting in the formation of shorter chains, followed by a reaction between the newly formed ester end-groups and initially present hydroxyl chain ends. The acidolysis of PPVL with the diacids proved to be less effective; in the case of the acidolysis of PPVL with 10 mol-% isophthalic acid, less than 1% of the diacid was incorporated in the polymer chains and a decrease in the logarithmic viscosity number of only 22% was found. Both the high stability of the ester bond in PPVL towards acids in general and the heterogeneity of these systems were supposed to cause the behaviour of PPVL with respect to acidolysis. The results concerning the interchange reactions with PPVL were compared with studies on other polyesters

Pivalolactone, 2. Copolyester synthesis via interchange reactions with polypivalolactone

The synthesis of copolyesters via interchange reactions of polypivalolactone (PPVL) with several compounds was studied. The synthetical procedures are two-stage melt processes: in the first stage ester bonds in the polymer chain are cleaved and new groups are incorporated in the polymer chain, while in the second step condensation of the end-groups formed occurs. For the synthesis of copolymers, three procedures were used, with tetrabutyl orthotitanate as a catalyst. PPVL was heated with equimolar mixtures of bisphenol-A diacetate (BPAac) and terephthalic acid (TA), but no copolymers were formed; instead, polycondensation of BPAac with TA occurred, leaving the PPVL unaffected. From PPVL and mixtures of BPAac and dimethyl terephthalate (DMT) polymers were obtained which contained a significant amount of copolymeric sequences. However, most of the polymeric chains consisted of PPVL and poly(bisphenol-A terephthalate) blocks. Random copolymers with thermal stability were obtained after heating PPVL with bisphenol-A polycarbonate and DMT. The latter process was studied in detail by IR, DSC, and solubility and selective degradation tests. Based on the results of these studies, the reactions occurring during the three procedures were discussed

Pivalolactone, 3. Reactive blending of polypivalolactone with polycarbonate

The occurrence of interchange reactions during heating of polypivalolactone (PPVL) with three polymers and their influence on the blend properties were studied. Physical blends of PPVL and bisphenol-A polycarbonate (PC) were found to be immiscible. By heating of PPVL/PC blends in the melt at 280°C, in diphenyl ether at 260°C and in a twin-screw extruder (TSE) at 280°C partial formation of copolymers was observed, provided that tetrabutyl orthotitanate (TnBT) was added. After heating of equimolar PPVL/PC blends in a TSE for 15 min, a PPVL-PC block copolymer could be isolated containing 25 mol-% pivalolactone (PVL) units. The results from thermal analyses indicated that PPVL/PC blends had become more miscible, due to the presence of copolymers formed by interchange reactions. After heating of equimolar mixtures of PPVL and a polyarylate (PAr) in a TSE for 15 min, PPVL-PAr copolymers with 5 mol-% PVL units could be isolated. Probably due to this low degree of interchange, no effect on the miscibility of the initially immiscible PPVL/PAr blends could be observed. PPVL/poly(butylene terephthalate) (PBT) blends, obtained after heating in a TSE, decomposed at a temperature between the melting temperatures of PPVL and PBT, indicating that interchange reactions may have occurred

Unsupervised Cross-Lingual Information Retrieval Using Monolingual Data Only

We propose a fully unsupervised framework for ad-hoc cross-lingual information retrieval (CLIR) which requires no bilingual data at all. The framework leverages shared cross-lingual word embedding spaces in which terms, queries, and documents can be represented, irrespective of their actual language. The shared embedding spaces are induced solely on the basis of monolingual corpora in two languages through an iterative process based on adversarial neural networks. Our experiments on the standard CLEF CLIR collections for three language pairs of varying degrees of language similarity (English-Dutch/Italian/Finnish) demonstrate the usefulness of the proposed fully unsupervised approach. Our CLIR models with unsupervised cross-lingual embeddings outperform baselines that utilize cross-lingual embeddings induced relying on word-level and document-level alignments. We then demonstrate that further improvements can be achieved by unsupervised ensemble CLIR models. We believe that the proposed framework is the first step towards development of effective CLIR models for language pairs and domains where parallel data are scarce or non-existent

Performance of the Product of Three Nakagami-m Random Variables

An output signal from a multi-section wireless relay communication system is equal to the product of the signal envelopes from individual sections. In this paper, a three-sections relay system is considered in the presence of Nakagami-m fading at each section. First, random variable (RV) is formed as the product of three Nakagami-m RVs. For such product, the moments are determined in the closed forms. The first moment is the mean of the signal; the second moment is the average power of the signal, and the third moment is skewness. Then, the Amount of Fading (AoF) is calculated. AoF is a measure of the severity effect of fading in a particular channel model. Besides, all system performance are shown graphically and the parameters influence has been analyzed and discussed

Bio-SimVerb

Evaluation Dataset: Samples/words in Bio-SimVerb (verbs) and Bio-SimLex (nouns) are collected from a pre-processed PubMed Central Open Access subset (PMC). POS tags and tokens in this resource are generated using the BLLIP constituency parser, trained on a biomedical corpus. The resource covers over 1.4M full articles with more than 388M parsed sentences. Details can be referred in Section 3 of the paper: Bio-SimVerb and Bio-SimLex: wide-coverage evaluation sets of word similarity in biomedicin

Modeling Language Variation and Universals: A Survey on Typological Linguistics for Natural Language Processing

Linguistic typology aims to capture structural and semantic variation across the world’s languages. A large-scale typology could provide excellent guidance for multilingual Natural Language Processing (NLP), particularly for languages that suffer from the lack of human labeled resources. We present an extensive literature survey on the use of typological information in the development of NLP techniques. Our survey demonstrates that to date, the use of information in existing typological databases has resulted in consistent but modest improvements in system performance. We show that this is due to both intrinsic limitations of databases (in terms of coverage and feature granularity) and under-utilization of the typological features included in them. We advocate for a new approach that adapts the broad and discrete nature of typological categories to the contextual and continuous nature of machine learning algorithms used in contemporary NLP. In particular, we suggest that such an approach could be facilitated by recent developments in data-driven induction of typological knowledge.</jats:p

PSYCHOLOGICAL EFFECTS OF "DOUBLE CRISIS" (COVID-19 PANDEMIC AND EARTHQUAKES) ON CROATIAN MEDICAL STUDENTS

Introduction: In 2020. the COVID-19 pandemic presented an additional source of stress and anxiety not just to the general population but also to medical students who are, even under normal circumstances, constantly under pressure due to demanding student duties. In addition, they experienced a series of devastating earthquakes in and around the Zagreb region which altogether could have had compromised their psychological well-being. The aim of this review was to evaluate the psychological effects of these two natural disasters on the mental health of Croatian medical students. Results: According to standardized questionnaires for depression and anxiety evaluation, 75.3% of students were anxious and 65.2% were depressive during to outcomes was observed regarding genders, but it was found that first year students had a significantly higher anxiety score than older ones. Conclusion: In such stressful situations, we should emphasize the importance of mental health not just of healthcare workers, but also of medical students in order to prevent serious psychological consequences and to alleviate the negative motivation and their educational process

PSYCHOLOGICAL EFFECTS OF "DOUBLE CRISIS" (COVID-19 PANDEMIC AND EARTHQUAKES) ON CROATIAN MEDICAL STUDENTS

Introduction: In 2020. the COVID-19 pandemic presented an additional source of stress and anxiety not just to the general population but also to medical students who are, even under normal circumstances, constantly under pressure due to demanding student duties. In addition, they experienced a series of devastating earthquakes in and around the Zagreb region which altogether could have had compromised their psychological well-being. The aim of this review was to evaluate the psychological effects of these two natural disasters on the mental health of Croatian medical students. Results: According to standardized questionnaires for depression and anxiety evaluation, 75.3% of students were anxious and 65.2% were depressive during to outcomes was observed regarding genders, but it was found that first year students had a significantly higher anxiety score than older ones. Conclusion: In such stressful situations, we should emphasize the importance of mental health not just of healthcare workers, but also of medical students in order to prevent serious psychological consequences and to alleviate the negative motivation and their educational process