The work presents an evidence in support of chain length dependent termination during cross-linking polymerization. It is based on the behavior of the ratio of the bimolecular termination coefficient
...k
t
b
to propagation rate coefficient
k
p during the after-effect of a photo-induced polymerization. The chain-length dependence was manifested by a decrease of the
k
t
b
/
k
p
ratio with the increase in dark reaction time faster than that resulting from the conversion increase. Two monomethacrylate/dimethacrylate and one dimethacrylate/dimethacrylate systems were chosen, which enabled to study the chain-length dependence as a function of cross-link density of the polymer being formed and physical properties of the initial composition. The ratios of the polymerization rate coefficients
k
t
b
/
k
p
were calculated for various postpolymerization processes as a function of dark reaction conversion using the mixed termination model (concerning the bimolecular and monomolecular termination occurring parallel). A rapid drop of the
k
t
b
/
k
t
ratio during the dark reaction was observed at the beginning of the after-effect and in slightly cross-linked systems suggesting a significant chain length dependence, but when the cross-link density increased with conversion or with cross-linking agent concentration in the feed, the drop of the
k
t
b
/
k
t
ratio became much slower indicating that the chain length dependence decreased.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The photo-induced copolymerization of dodecyl methacrylate (DDM) with five oxyethylene glycol dimethacrylates (OEGDM) was investigated. Effects of the monomer ratio and of the length of ...dimethacrylate spacer group on the polymerization kinetics, the extent of the after-effect and pendant double bond content in the polymerization product were studied and the reactivity ratios estimated. For systems containing OEGDM with short spacers between the methacrylate groups,
R
p
ma
x
reached the highest value at certain monovinyl/divinyl monomer ratio. This phenomenon was discussed in terms of the behavior of the reaction diffusion parameter (as a function of monomer ratio and conversion). Determination of the reactivity ratios by five calculation methods showed that
r
1 (DDM) values were lower than 1 and the
r
2 (OEGDM) values were higher than 1 indicating that the polymer formed at the beginning of the reaction is more densely cross-linked than that formed in the final reaction stages.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Laisvo darbuotoju judejimo ir Europos be sienu ideja buvo igyvendinta remiantis Vieningos Europos aktu. Pazanga siekiant laisvo asmenu judejimo buvo palyginti leta, nes Europos Sajungos valstybes ...nenoriai atvere savo vidaus sienas treciuju valstybiu pilieciams, taciau ju atverimas buvo gyvybiskai butinas siekiant sukurti viena rinka. Todel 1995 m. isigaliojus Sengeno konvencijai buvo panaikinti patikrinimai tarp vidaus sienu ir sugrieztinta isoressienu kontrole. Europos Komisijos iniciatyva Bendrijos teise ir Sengeno konvencija buvo susietos priemus papildoma Amsterdamo sutarties protokola. Acquis Schengen tapus Bendrijos teises dalimi, stojancioms i Europos Sajunga salims, taip pat ir Lenkijai, buvo numatyta pareiga igyvendinti Sengeno nuostatas. Per acquis Schengen nuostatu igyvendinimo prizme butu galima izvelgti tris Lenkijos ir Europos Sajungos santykiu laikotarpius kai: 1) Lenkijos pilieciai buvo laikomi uzsienieciais, t. y. treciosios valstybes pilieciais; 2) Lenkijos pilieciai jau nebera laikomi treciosios valstybes pilieciais, bet Sengeno konvencijos nuostatos dar nera privalomos Lenkijai; 3) Lenkijos pilieciai nebus laikomi uzsienieciais, t. y. treciosios salies pilieciais, ir Lenkija taikys acquis Schengen. Nors istojus i Europos Sajunga Lenkijos pilieciai jau nebera laikomi treciuju valstybiu pilieciais, laisvai ir nekontroliuojamai kirsti senuju Europos Sajungos valstybiu nariu sienas jie gales tik tada, kai Lenkija prades taikyti acquis Schengen.
Objectives. Resin-modified glass ionomer cements (RMGI) are hybrid materials prepared by incorporation of polymerizable components (typically 2-hydroxyethyl methacrylate (HEMA) with possible addition ...of multifunctional methacrylates) into a conventional acid–base mixture (a polymeric acid with powdered calcium fluoro-aluminosilicate glasses). During setting, the photopolymerization process and the acid–base reaction affect each other. The aim of this work was to examine the effect of a 45% aqueous solution of poly(acrylic acid) (PAA) and the liquid component of a commercial glass ionomer cement on HEMA and triethyleneglycol dimethacrylate (TEGDMA) photopolymerization.
Methods. The polymerization was initiated by 2,2-dimethoxy-2-phenylacetophenone (DMPA) and camphorquinone (CQ)/coinitiator system. The reaction course was monitored under Ar and air by isothermal differential scanning calorimetry.
Results. The main effect of addition of polyacid solution (PAA and commercial) up to 10
wt% to HEMA on the polymerization initiated with DMPA was earlier onset of autoacceleration. For the process initiated by the CQ-based system, the addition of 5
wt% of PAA solution strongly accelerated the polymerization and increased the conversion, both in Ar atmosphere as well as in air. TEGDMA photopolymerization was not influenced or slightly retarded by the presence of 3
wt% of PAA solution (the upper limit of solubility), depending on the initiating system used.
Significance. Under initiation conditions used in curing of commercial dental products (CQ-based two component initiating system), the presence of polyacid-aqueous solution in HEMA-based photocurable component increases markedly the polymerization rate and the conversion both in Ar atmosphere as well as in air. This result contributes to a characterization of the setting process of RMGIs.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
PDF
