Poly(vinyl alcohol) (PVA) was cross-linked with maleic anhydride (MA) and 2,5-furandicarboxylic acid (FDCA) under conventional heating (CH) and microwave irradiation (MW) to obtain different membranes of PVA/MA and PVA/FDCA modified by the cross-linking agent concentration, reaction time and type of activation method. The influence of the latter parameters, was studied by comparing the swelling degree of the membranes, swelling kinetics, solubility parameter (δ), Flory interaction parameter (χ 12 ), molecular weight between crosslinks (M c ) and the fractional free volume (FFV). Conditions to cross-link the membranes were chosen from swelling percentage tests and the samples with the lowest water uptake were characterized by means of Fourier transform infrared spectroscopy (FTIR), 13 C nuclear magnetic resonance ( 13 C NMR), thermogravimetric analysis (TGA) and atomic force microscopy (AFM). MW activation used from 23 to 30 minutes to cross-link the membranes, meanwhile, conventionally heated samples were introduced in an oven for 120 min, meaning that MW activation provided high reaction yields in shorter times. For both activation methods, FTIR spectra recorded peaks in an interval of 1706-1713 cm À 1 , and 13 C NMR presented signals from 162 to 171 ppm, meaning that carbonyl groups from esters were formed. AFM images showed that the rise in roughness was not necessarily directly proportional to the increase in the concentration of cross-linking agent.

<p>Este año, la Química Macromolecular celebra su primer centenario. En 1920, el químico orgánico Hermann Staudinger, publicó su célebre artículo <em>Über Polymerisation </em>(Sobre Polimerización), proponiendo la existencia de largas cadenas de muy alto peso molecular, en las cuales los átomos se mantienen juntos por medio de uniones químicas covalentes. En el transcurso de estos 100 años, la Química de los Polímeros ha avanzado rápidamente, siendo reconocida hoy como una de las ramas más activas en investigación, desarrollando nuevos productos útiles con una amplia gama de aplicaciones.</p><p>Este trabajo trata sobre aspectos relevantes de la historia de la Química Macromolecular. Se mencionan algunos de los nombres de los investigadores más importantes que ayudaron a desarrollar esta ciencia durante el siglo XX y también se nombran algunos de los profesores que iniciaron la enseñanza de los polímeros en México, específicamente en la Universidad Nacional Autónoma México (UNAM).</p>

Science Education International ¦ Volume 32 ¦ Issue 2 107 ORIGINAL ARTICLE INTRODUCTION The problem-based learning (PBL) methodology is a student-centered approach that is related to the learning process that occurs when students deal with real world problems, while working in teams to find and develop a solution, with teachers/instructors acting as facilitators (Nagarajan and Overton, 2019). Some elements seem to be common to PBL: Learning is student centered (as mentioned before), problems are structured and authentic, teachers act as advisors, and students work in small groups (Cowden and Santiago, 2016). Although, the elements are in constant interaction, students are responsible of their learning, implying that they have the main role in the cognitive process and should work actively, in group, to solve a problem. On the other hand, instructors act as coaches, they incite group discussion, and they are in charge of monitoring the process. Students are the main characters, since PBL methodology emerged from constructivist learning theories and it was developed as an alternative to conventional teaching (Loyens et al., 2006). Constructivism suggests that humans build knowledge from their experiences and, contrary to traditional education, where students receive knowledge like empty vessels to be filled, in constructivist, students are encouraged to confront what they know (Bada and Olusegun, 2015). It is evident that, long-term memorability is enhanced by PBL, because it fosters the utilization of previous knowledge to solve a new problem and demands students to put in practice what they have already been taught, therefore, facilitating the comprehension of the concepts (Schmidt et al., 2011). Other benefits that come along with PBL include the improvement of student’s creative thinking, self-regulated skills, and self-evaluation (Jansson et al., 2015; Yoon et al., 2014). Therefore, to improve chemistry student’s learning experience, the PBL approach can be used for a better comprehension of the importance of Green Chemistry. According to the U.S. Environmental Protection Agency (EPA, 1990), Green Chemistry is the design of chemical products and processes that reduce or eliminate the use or generation of hazardous substances; this designing process can be assisted by the Twelve Principles of Green Chemistry (Lancaster, 2002). The principles are qualitative, and their aim is to minimize the impact of chemical activities on human health and environment without compromising the chemical process (Ribeiro and Machado, 2013). There is a commitment to green chemistry education (Armstrong et al., 2018), and efforts have been made to implement it, at the undergraduate level (Timmer et al., 2018; Kennedy, 2016; Manchanayakage, 2013), but there is an uneven development of green chemistry curricular materials, since there have been few comprehensive reforms for general chemistry lecture or laboratory curricula (Armstrong et al., 2019). For example, Green Chemistry has not been covered extensively by chemistry A problem-based learning (PBL) methodology was implemented to a project, whose main objectives were to discuss and apply the Twelve Principles of Green Chemistry to the study of poly(vinyl alcohol)’s cross-linking reaction with dicarboxylic acids. The five participating students were oriented to be responsible for their own learning and the professor participated as an advisor. The problem was proposed and students planned all their activities to accomplish the objectives and goals, reviewed recent information in scientific literature and summarized it, made experimental work, prepared written reports, and were evaluated in seminars. The results obtained by the students were assessed through the generation of a final report and also with a final oral presentation in front of faculty members. The experience lived by the collaborative workgroup during the development and execution of the project, is described. This research is an example of how the PBL methodology can motivate the active participation of students when solving problems. The next step is to introduce this tool to teachers and students of other undergraduate courses or laboratories, since it causes a difference in the way education is being perceived in our university, because it emphasizes the application and understanding of concepts over simple memorization.