CONTROL OF KEY POLYMER PROPERTIES VIA REVERSIBLE ADDITION-FRAGMENTATION CHAIN TRANSFER IN EMULSION POLYMERIZATION

Free radical emulsion polymerization (FRP) is widely adopted in industry due to its applicability to a wide range of monomers. Despite its many benefits and wide spread use, the fast chain growth and the presence of rapid irreversible termination impose limitations with respect to the degree of control in FRP. Furthermore, producing block copolymers and polymers with complex structures via FRP is not feasible. Closer control of macromolecular chain structure and molar mass, using novel polymerization techniques, is required to synthesize and optimize many new polymer products. Reversible addition fragmentation chain transfer (RAFT)-mediated polymerization is a novel controlled living free radical technique used to impart living characters in free radical polymerization. In combination with emulsion polymerization, the process is industrially promising and attractive for the production of tailored polymeric products. It allows for the production of particles with specially-tailored properties, including size, composition, morphology, and molecular weights. The mechanism of RAFT process and the effect of participating groups were discussed with reviews on the previous work on rate retardation. A mathematical model accounting for the effect of concentrations of propagating, intermediate, dormant and dead chains was developed based on their reaction pathways. The model was combined with a chain-length dependent termination model in order to account for the decreased termination rate. The model was validated against experimental data for solution and bulk polymerizations of styrene. The role of the intermediate radical and the effect of RAFT agent on the chain length dependent termination rate were addressed theoretically. The developed kinetic model was used with validated kinetic parameters to assess the observed retardation in solution polymerization of styrene with high active RAFT agent (cumyl dithiobenzoate). The fragmentation rate coefficient was used as a model parameter, and a value equal to 6×104 s-1 was found to provide a good agreement with the experimental data. The model predictions indicated that the observed retardation could be attributed to the cross termination of the intermediate radical and, to some extent, to the RAFT effect on increasing the average termination rate coefficient. The model predictions showed that to preserve the living nature of RAFT polymerization, a low initiator concentration is recommended. In line with the experimental data, model simulations revealed that the intermediate radical prefers fragmentation in the direction of the reactant. The application of RAFT process has also been extended to emulsion polymerization of styrene. A comprehensive dynamic model for batch and semi-batch emulsion polymerizations with a reversible addition-fragmentation chain transfer process was developed. To account for the integration of the RAFT process, new modifications were added to the kinetics of zero-one emulsion polymerization. The developed model was designed to predict key polymer properties such as: average particle size, conversion, particle size distribution (PSD), and molecular weight distribution (MWD) and its averages. The model was checked for emulsion polymerization processes of styrene with O-ethylxanthyl ethyl propionate as a RAFT based transfer agent. By using the model to investigate the effect of RAFT agent on the polymerization attributes, it was found that the rate of polymerization and the average size of the latex particles decreased with increasing amount of RAFT agent. It was also found that the molecular weight distribution could be controlled, as it is strongly influenced by the presence of the RAFT based transfer agent. The effects of RAFT agent, surfactant (SDS), initiator (KPS) and temperature were further investigated under semi-batch conditions. Monomer conversion, MWD and PSD were found to be strongly affected by monomer feed rate. With semi-batch mode, Mn and increased with increasing monomer flow rate. Initiator concentration had a significant effect on PSD. The results suggest that living polymerization can be approached by operating under semi-batch conditions where a linear growth of polymer molecular weight with conversion was obtained. The lack of online instrumentation was the main reason for developing our calorimetry-based soft-sensor. The rate of polymerization, which is proportional to the heat of reaction, was estimated and integrated to obtain the overall monomer conversion. The calorimetric model developed was found to be capable of estimating polymer molecular weight via simultaneous estimation of monomer and RAFT agent concentrations. The model was validated with batch and semi-batch emulsion polymerization of styrene with and without RAFT agent. The results show good agreement between measured conversion profiles by calorimetry with those measured by the gravimetric technique. Additionally, the number average molecular weight results measured by SEC (GPC) with double detections compare well with those calculated by the calorimetric model. Application of the offline dynamic optimisation to the emulsion polymerization process of styrene was investigated for the PSD, MWD and monomer conversion. The optimal profiles obtained were then validated experimentally and a good agreement was obtained. The gained knowledge has been further applied to produce polymeric particles containing block copolymers. First, methyl acrylate, butyl acrylate and styrene were polymerized separately to produce the first block. Subsequently, the produced homopolymer attached with xanthate was chain-extended with another monomer to produce block copolymer under batch conditions. Due to the formation of new particles during the second stage batch polymerization, homopolymer was formed and the block copolymer produced was not of high purity. The process was further optimized by operating under semi-batch conditions. The choice of block sequence was found to be important in reducing the influence of terminated chains on the distributions of polymer obtained. It has been found that polymerizing styrene first followed by the high active acrylate monomers resulted in purer block copolymer with low polydispersity confirmed by GPC and H-NMR analysis

Longitudinal Linkages Between Father and Mother Autonomy Support and Adolescent Problem Behaviors: Between-Family Differences and Within-Family

Despite existing evidence on negative associations between parental autonomy support and children’s internalizing and externalizing problem behavior, it is difficult to draw conclusions on the effect that parents’ autonomy support has on children’s problem behavior. This study contributed to the existing literature by unraveling the temporal ordering of parental autonomy support and adolescent problem behavior. In addition, this study examined whether these linkages differed by parent’s sex, child sex, and reporter of autonomy support. Data of 497 adolescents (mean age at T1 = 13.03 years, percentage male = 56.9) and their parents from six annual waves of the Dutch study Research on Adolescent Development And Relationships (RADAR) were used. The results showed that stable differences between families explained most linkages between autonomy support and problem behavior. Adolescents with fewer problem behaviors have fathers (both child- and parent-reported) and mothers (only child-reported) who are more autonomy supportive. The results did not differ between boys and girls. The findings suggest that prior studies may have overstated the existence of a causal effect of parental autonomy support on adolescent problem behavior

Daily Identity Dynamics in Adolescence Shaping Identity in Emerging Adulthood

According to identity theory, short-term day-to-day identity exploration and commitment processes are the building blocks for long-term development of stable commitments in emerging adulthood. This key assumption was tested in a longitudinal study including 494 individuals (43% girls, Mage T1 = 13.31 years, range 11.01–14.86 years) who were followed from adolescence into emerging adulthood, covering ages 13 to 24 years. In the first five years, adolescents reported on their daily identity processes (i.e., commitment, reconsideration and in-depth exploration) across 75 assessment days. Subsequently, they reported on their identity across four (bi-) annual waves in emerging adulthood. Findings confirmed the existence of a dual-cycle process model of identity formation and identity maintenance that operated at the within-person level across days during adolescence. Moreover, individual differences in these short-term identity processes in adolescence predicted individual differences in identity development in emerging adulthood. Specifically, those adolescents with low daily commitment levels, and high levels of identity reconsideration were more likely to maintain weak identity commitments and high identity uncertainty in emerging adulthood. Also, those adolescents characterized by stronger daily changes in identity commitments and continuing day-to-day identity uncertainty maintained the highest identity uncertainty in emerging adulthood. These results support the view of continuity in identity development from short-term daily identity dynamics in adolescence to long-term identity development in emerging adulthood

Daily Identity Dynamics in Adolescence Shaping Identity in Emerging Adulthood: An 11-Year Longitudinal Study on Continuity in Development

According to identity theory, short-term day-to-day identity exploration and commitment processes are the building blocks for long-term development of stable commitments in emerging adulthood. This key assumption was tested in a longitudinal study including 494 individuals (43% girls, Mage T1 = 13.31 years, range 11.01–14.86 years) who were followed fro

Parental Age in Relation to Offspring's Neurodevelopment

Objective: Advanced parenthood increases the risk of severe neurodevelopmental disorders like autism, Down syndrome and schizophrenia. Does advanced parenthood also negatively impact offspring’s general neurodevelopment? Method: We analyzed child-, father-, mother- and teacher-rated attention-problems (N = 38,024), and standardized measures of intelligence (N = 10,273) and educational achievement (N = 17,522) of children from four Dutch population-based cohorts. The mean age over cohorts varied from 9.73–13.03. Most participants were of Dutch origin, ranging from 58.7%-96.7% over cohorts. We analyzed 50% of the data to generate hypotheses and the other 50% to evaluate support for these hypotheses. We aggregated the results over cohorts with Bayesian research synthesis. Results: We mostly found negative linear relations between parental age and attention-problems, meaning that offspring of younger parents tended to have more attention problems. Maternal age was positively and linearly related to offspring’s IQ and educational achievement. Paternal age showed an attenuating positive relation with educational achievement and an inverted U-shape relation with IQ, with offspring of younger and older fathers at a disadvantage. Only the associations with maternal age remained after including SES. The inclusion of child gender in the model did not affect the relation between parental age and the study outcomes. Conclusions: Effects were small but significant, with better outcomes for children born to older parents. Older parents tended to be of higher SES. Indeed, the positive relation between parenta

On Imbalance of Impulse Control and Sensation Seeking and Adolescent Risk

Heterogeneity in development of imbalance between impulse control and sensation seeking has not been studied until now. The present study scrutinized this heterogeneity and the link between imbalance and adolescent risk. Seven-wave data of 7,558 youth (50.71% males; age range from 12/13 until 24/25) were used. Three developmental trajectories were identified. The first trajectory, “sensation seeking to balanced sensation seeking”, included participants with a higher level of sensation seeking than impulse control across all ages. The second trajectory, “moderate dominant control”, included participants showing moderate and increasing impulse control relative to sensation seeking across all ages. The third trajectory, “strong late dominant control”, included participants showing the highest level of impulse control which was about as strong as sensation seeking from early to middle adolescence and became substantially stronger from late adolescence to early adulthood. Although the systematic increase of impulse control in all subgroups is in line with both models, neither of these combined trajectories of control and sensation seeking was predicted by the Dual Systems Model or the Maturational Imbalance Model. Consistent with both models the “sensation seeking to balanced sensation seeking” trajectory showed the highest level of substance use. It can be concluded that, even though both theories adequately predict the link between imbalance and risk,

Development of parent-adolescent relationships: Conflict interactions as a mechanism of change

Adolescence is a period of rapid biological and psychosocial changes, which have a salient impact on parent–child relationships. Parents and adolescents have to reorganize responsibilities and move toward a more egalitarian relationship. Although conflicts between parents and children become more frequent and more intense during adolescence, these conflicts are also thought to be a means to negotiate relational changes. The short‐term dyadic processes that occur during conflict interactions are important in the development of parent–adolescent relationships. Parent–adolescent dyads with more emotional variability during conflict interactions tend to adapt effectively and reorganize their relationships in response to the developmental needs of adolescents. Thus, parent–adolescent conflicts are adaptive for relational development when parents and adolescents can switch flexibly between a range of positive and negative emotions

Families with Adolescent Children: Development in Relationships

Contains fulltext : mmubn000001_388632402.pdf (publisher's version ) (Closed access)RU Radboud Universiteit Nijmegen, 21 januari 2003Promotores : Lieshout, C.F.M. van, Aken, M.A.G. van140 p